Some other Pros: Easier to test individual pieces. Easier to benchmark. More usage == more/better features/functionality for everyone. Easier for people to contribute to without having to know the full stack. I think most people agree that decoupled, reusable modules are a good thing in general as an abstract concept, but, of course, specifics matter.

I think this needs a bit more explanation. AIUI, the primary cause for concern is that by making something a module, you are taking a private, internal API of Solr's and now making it a public API that must be maintained (and backwards maintained) which could slow down development as one now needs to be concerned with more factors than you would if it were merely an implementation detail in Solr.

Other Cons: The concern was that Solr just becomes an uninteresting, empty shell that glues together modules. (I don't agree, but wanted to present what I have heard)