I guess I'm worrying about wasting my time and the time of exhibitors, who at the end of the day are there to sell stuff to people with money - not to not sell stuff to people who don't have money.
If there's one thing worse than chatting to people on your trade show stand who aren't going to buy, it's having nobody on the stand at all.
Provided you make it clear you aren't buying, people running stands will normally be quite happy to chat to you when there are no real buyers in prospect, so long as you evaporate smartly when a real punter wanders into view!