Pre-war alliance strategy
Why did the Star Kingdom of Manticore seek so many weak, spatially dispersed allies?
First, almost all of those allies were acquired prewar. They were part of a system of interlocking bases which were designed to support scouting forces and to threaten the flanks and rear of any conventional (that is, based on prewar strategic thinking, however outmoded it may have proven in practice) advance against the Star Kingdom. They were also sought as a means of slowing the conventional Peep method of frontier expansion.
The People's Republic had used two different basic techniques for expansion. One, obviously, was the conquest of star systems with the wealth and/or industrial potential to help prop up the People's Republic's shaky finances. But they had actually acquired a greater total number of the star systems which were eventually incorporated into the People's Republic using their second technique, which was which you might call a hostile takeover. By enlisting star systems in their economic and military sphere of interest, they had created a sort of mini-version of the Office of Frontier Security. This was a steady, ongoing activity which seldom reached the level of overt Peep military action. Mind you, the Peep weren't at all averse to fomenting local domestic unrest to "encourage" the local government to request military assistance from their good friends and neighbors in Nouveau Paris. Nor were they averse to systematically assassinating political leaders in the star systems in question until they managed to get a local government which was willing to "request military assistance."
What the Star Kingdom of Manticore was doing under the foreign-policy initiated by King Roger and continued and enhanced by Queen Elizabeth was to seek offsetting military bases, to create a barrier or strategic glacis which the Peeps would have to penetrate (bearing in mind that we're talking about the prewar strategic concept of star-by-star advances), and to bring the remaining uncommitted star nations between them and the Peeps into the Manticoran economic and military sphere of interest in order to deny them to the Peeps. And, in addition, the Star Kingdom was deliberately staking out its claim to the position of the leader of a united resistance to Peep expansionism. This was both intended to encourage local star nations to stand up to Havenite intrusions, and to slow the rate at which the Peeps were approaching areas which were vital to Manticore's strategic interests. Hopefully, the blocking of the Peeps' OFS-style expansion could be counted upon to throw a monkey wrench into the overall unfolding of the DuQuesne Plan. That, hopefully, would at least cause some dislocation and delay in what both Roger and Elizabeth saw as the eventual, inevitable attack on Manticore itself.
The relationships established with these minor interstellar powers did contribute at least something to the bottom line of the Star Kingdom's interstellar balance of trade (although much of that was offset by the additional cost to the Star Kingdom's exchequer in transfer payments and military spending to bolster those allies' ability to look after themselves). In a lot of ways, you could think of this gradual expansion towards one another of the two future adversaries' spheres of interest as the equivalent of establishing the Warsaw Pact and NATO. I'm not saying the analogy is all that precise; I'm simply suggesting that it may be useful as a model from our own recent historical experience when it comes to trying to appreciate the prewar thinking of the two sides.
I've been asked why the Star Kingdom didn't encourage a lot of their minor allies to essentially declare nonbelligerent status once the actual war broke out, since this would have relieved the Alliance of the necessity of defending smaller star nations which weren't net positive contributors to the Alliance's military capabilities. There are several reasons for this.
First, the Peeps wouldn't have cared. If a star nation had ever been a Manticoran ally -- and especially if that's star system might have examples of modern Manticoran military hardware which could be captured and taken apart by the Peeps -- then a declaration of nonbelligerence would have been about as much protection as a screen door on an airlock. And if anyone thinks that concerns over public opinion, whether their own domestic public opinion, or that of the Manticoran Alliance, or that of the Solarian League, would have made any difference to the people responsible for formulating Peeps' policy, I have some bottom land about a hundred miles east of Florida I'd like to sell them.
Secondly, the Star Kingdom's honor was involved. One reason Honor Harrington was prepared to put it all on the line to defend Yeltsin's Star against Masada in The Honor of the Queen was because she realized that the Star Kingdom's good faith and reliability were already on the line. One of the more unforgivable sins of the High Ridge Government is that High Ridge squandered the Star Kingdom's well-deserved reputation for fidelity to its word and to the observation of its responsibilities to its allies. King Roger and Queen Elizabeth had both been extraordinarily careful, both because it was part of their personalities and because they well understood the realities of interstellar diplomacy, to never go back on or violate a promise to another star nation. I believe the pragmatic reasons they felt that way (and why) are amply demonstrated in what happened in Crown of Slaves and the general disarray of the Manticoran Alliance in the wake of High Ridge's version of foreign policy. Much of the friction which was observable between the Star Kingdom and its allies in At All Costs was the direct result of High Ridge's disastrous effect on other star nations' perception of the reliability of the Star Kingdom's word. Elizabeth, Cromarty, and the Admiralty understood how vital it was for the Star Kingdom to be perceived as a reliable partner by potential allies and as an honest negotiator when it came to dealing with other major star nations like the Andermani Empire, for example.
The bottom line is that the Star Kingdom was looking for allies in all of those dispersed, relatively weak star nations, as part of a prewar strategy which was aimed at delaying the war between Manticore and Haven long enough for the military buildup initiated by King Roger and continued and accelerated under Queen Elizabeth to reach a point at which the Star Kingdom had a hope of victory. And it was a policy which was pursued under a prewar strategic concept of operations which proved to be obsolete once the Star Kingdom's new weaponry started coming forward.
By the way, folks. The mere fact that the Star Kingdom of Manticore under Cromarty and Elizabeth did something, or pursued some specific policy, doesn't necessarily mean that the Manties were right when they did it. It means that some pretty darn smart people, operating within their understanding and perception of the situation, thought they were doing the right thing. And, overall, the Manties did a heck of a lot more "right things" than they did "wrong things." That doesn't mean that when I was writing the books, and considering the future directions of strategy and warfare in the Honorverse, that I agreed with them about the correctness of some of their strategic decisions. I didn't always choose to tell you that I thought something was a Bad Idea, but I believe that if you go back and look at the books, you'll see several things working out suboptimally, let's say, even if the people responsible for deciding to do them thought they were a Good Idea.