They were a really clear indicator that the occasional companies that hired them seriously didn't understand their audience, and hadn't brought anybody who knew anything technical to their booth, probably not even any marketing people who understood the product, so you could pretty much skip them, because they were pretty much always useless as well as unprofessional.
On the other hand, you can totally bribe us with chocolate or especially coffee, and we might sit through your silly magician act for a raffle ticket for an iThing as long as there was technical content at your booth, and we'll pick up blinky tchotchkes with your logo on them. The woman I'd rather talk to at your booth is the one who developed the cool product, or can explain it well.
When my company's been at trade shows in the area, about half their staff are booth-running professionals, rather than product-related, from the people who set the thing up and make sure all the marketing content is there to the people who herd customers in, figure out what they're interested in (even if it's just at the buzzword level), bring them over to the right part of the booth or find the right person if they need to, scan your contact info, get the speakers on and off the stage, etc., and about half are either main-office or local people who know something about whatever we're trying to sell. They seem to do a good job on the mechanics of it (I've occasionally ended up as local booth staff), and they're seriously good at respecting the audience.