A few thoughts.
AW is a non-profit, not a charity. Punters that are grown up enough to help themselves a bit by joining UKP, or even better, joining and posting reviews, are more deserving in my opinion. I don't feel any 'duty' to try to 'help' those that just read AW reviews.
As long as you take all reviews with a pinch of salt, and mostly ignore stupid or agenda-laden ones, then they can be a bit helpful, but no substitute for doing your own research. UKP reviews are the only ones I think are worth reading. I check AW reviews really only to see if she's got any (a different exercise, part of "does she exist?").
Positive reviews are as important as neutral and negative ones but, all else being equal, negative ones (about anything, not just prossie services) tend to be more useful. You'd be hard-pressed to see every well-reviewed prossie. Think of holiday reviews: loads of people will like any destination you can pick, but it is the complaints that are likely to tell you if, for you, something not so good would be a deal-breaker.
Fluffie reviews (from people who only rate prossies as 9 or 10 out of 10) are mostly helpful in the same way as AW, the "does she exist?" question. They can be entertaining and even sincere, but not that useful when a reviewer seems to have such little experience or ability/willingness to differentiate between good and bad. The claim it is "all down to good research" is unscientific and unfalsifiable: one cannot image the fluffy being negative unless the girl ripped him off or failed to turn up. Sometimes not even then.
Reviewer street-cred is the most important thing. Even on AW (confirming she exists) a reviewer who I recognise as genuine helps to confirm it is not just a PG-seller profile.
Why review? a) if you want to; b) if the forum has given you something at some point then it's a decent thing to do to give something back. Remember: without compromising your identity.