From my impression, there was much less congratulations postings and discussions taking place in the threads after the winners were announced.
I personally use the "New Posts" link almost exclusively when reading the forum, so unless someone has recently commented in one of the announcement threads, I may not have time to get far enough down the list to run into it. It's also easy to get the forum broken up into too many "buckets" but speaking from someone who ran a contest, having a separate forum was a help, and I put the contest info and Winners post in the forum for that contest.
As a participant and voter, it was also fantastic to have all the voting poll threads together in one forum, as it made it easy to go to that forum and quickly confirm that I had voted in everything that was available.
While it seems like it may be a great idea to have a central Winner's forum, I would be afraid that things would get too segmented, and I really like the idea of the winners for each contest being posted in the contest forum from an archive perspective.