In general, buying a shareware has some benefits, because when you buy something it's supposed to be good. But even when Aduim is free, the devs are so dedicated to Adium, that there's no reason to buy Proteus. I have tryied both, and I don't see much differences between both. The preferences might be different, but the overall look is similar, and I find Adium more customizable.
The Adium team has became more professional, they redisigned the site, so now the extras and forum are separated from the the main site. If you navigate a bit more through the site, you can see all the projected updates. Also, the frequent updates don't bother me. I actually find it exiting, seeing chances every few days (I also update to betas).
File transfers work perfect for me. In a version a few updates ago file transfers crashed Adium, but that was fixed already. I think webcam is due for version 0.9 if the tool necessary becomes available (not sure about that).
Mercury and aMSN actually send webcam. I haven't been able to do it myself because after Tiger and iWork '05 (not sure which one) iMovie won't detect my Logitech webcam, which is necessary for webcaming (at least that is what Mercury.to suggests).
Bottom line: There's no need to buy Proteus. For eyecandies, simple convos, file transfers use Adium. For webcam, audio, winks, custom emoticons, nudges, use Mercury or aMSN. Altough Mercury is more customizable now, it still looks ugly. And file transfers are still slow in Mercury.