There is definitely no need for a subwoofer with the BeoLab 5s - in fact they outperform any other subwoofer I've heard on the market in regards to control, maximum SPL output, musicality and extension (heck they go easily into subsonic frequencies).
There is however, an argument which says that by using BeoLab 5s, you are completely disregarding the '.1' channel encoded in the movie, and that the bass which is going to the main speakers isn't actually the same as the bass which would normally go to the subwoofer.
Then again - every movie I've seen with BeoLab 5s involved has been outstanding!