When should conferences precirculate papers?
I can't see this happening in any sort of large-scale conference. This would involve too much work and logistical planning on the part of everyone involved to be successful. It's hard enough to get abstracts for many conferences—let alone finished papers far enough in advance that people have time to read them!

In addition, this last point is another major obstacle: people don't have a lot of time to read all the papers that we're supposed to, let alone a bunch for a particular conference. The main reason why I would undertake that much work was if it were for a relatively specialized workshop in my personal field of endeavor.