One of the stimuli for this blog was my attendance at an IT for marketing exhibiton a couple of months ago. A whole exhibition hall that was full of attendees who'd given permisison to these stand-holders to market to them and no-one was touting for business. They just stood around and expected us to come to them.
Since then, I've been asking around at trade shows and the consensus is that it's worth having a presence (why, if you don't exploit it, I wonder?) and we do a lot of networking (with people you already know it seems). Michael Calder via Seth's blog does a valiant job of suggesting how this could be improved, but we'd still have to work our way through the miasma of mediocrity that palls across these shows to locate those stands which are doing it right.
I prefer to be more disruptive - let them pay for the expensive stands and I'll go to the shows and market myself to them. That way I'm in control.