The question is, does the policy prevent highly trained testing staff who are willing to donate their time to developers for free from helping developers (but for their own reasons wish to remain anonymous).

It's a trade off, and real name policies don't only exclude negative posters.

(To expand: I tested someone's js project on reddit just yesterday, offering my platform, multiple browsers, a decent writeup, images/screenshots etc. But my reddit account is fully anonymous. If my real name was required, s/he never would have gotten that free help)