From a post to ALT.BOOKS.DAVID-WEBER dated October 18, 1998:

Missile pods: how well can you fit pods/box launchers on the exterior of a hull?

    Not very well. There is very little of a conventional DD's hull surface which is not being used for something as things now stand. Laser clusters, counter-missile launchers, sensor arrays, etc., all eat up external surface, and the pod launchers (for the reasons cited above) are bulky enough to cover up vital areas/systems currently located there. Please note that even if you could fit the pods in there, their initial launch impetus is much lower than navies in HH's universe consider the minimum acceptable, which makes it very unlikely that the concept would ever be embraced.