a centurion is supposed to be as strong as a ranger vet from what I heard from characters but there not and here's a video to prove it.
http://www.youtube.com/watch?v=vSq1dSGYYFM&feature=channel_video_title

I think only one ranger died during the whole thing (not counting the 1 vs 3) so I think just for balancing reasons make the Centurions stronger.

hmm assassins are better.... the new patch really buffed them.