That's a good question/point of discussion, and the answer is IMHO "it depends", from the manufacturers'  but also the customers' side.

A specialized ball with a narrow range of use can make sense - there are extreme ends of the condition range that IMO call for special purpose pieces - and there are some alternative designs that can fill gaps in the "normal" range of an arsenal.

IMHO, many high performance pieces have become quite narrow in use due to very aggressive/responsive coverstocks and strong mass bias cores. The good old "strong all-around ball) seems to be on the wane?

On the other side, many mid-range balls offer a wider range of use. Not sp spectacular to behold on the lane, but rather steady performer with tested/reliable coverstocks and milder/more conservative core designs.

I think that there is a trench opening up betwenn these "groups" of balls, and big manufacturers promote this development through their marketing efforts.
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This is a separate issue from "would you post a negative review of a ball you were paid to throw?".  Feedback between testers and developers should be honest and open, no one would dispute that.  If I were a ball tester, I would do exactly that, most of us would.

But that's not the same thing as being given release-ready equipment and providing positive reviews.  I'm sure ball staffs are given pre-production samples, and there are certainly in-house testers with access to developmental equipment.  Samples that may or may not be further tweaked (or scrapped) before being mass produced are not the same as balls that have been put into mass-production and given to reviewers early.

The difference is the same as between beta versions of software and release versions.  If you were given a beta version, with feedback expected between the user and developer, you should be open and honest with your criticism.  Your criticism probably won't be shown to the public at large, though.  If you are given a release version, and want to continue to receive release versions for free, your opinion on the quality is not as meaningful.

SH