I am with Mark....Freeform performances by a band or individual when recorded live are great and no click track is needed or should be used as it loses the live feel especially with ad libs thrown in etc. That said when a work is being layered piece meal and built up using Midi or VST loops etc then a click track is essential. The different parts have to mesh and fit perfectly to the beat therefore the tempo must be constant and spot on. Sadly I have not met any drummers or other musicians who can play freeform and match a strict tempo spot on without the need for a click track. In fact a lot of them play to a click track when laying down a part and use it as a tool to keep perfect time albeit only in their head. That does not mean that they have to play like a robot.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055