Part of the attraction of the megacons is the dealers space. Pre-LargeRiverCorp it was difficult to locate and purchase esoterica and conventions were a convenient weekend opportunity to hunt down books, movies, props, pictures, costumes etc. For quite a few first-time congoers the social aspects of the convention came second to the access to hucksters and their wares. I helped out a games dealer at the one Dragon*Con I attended (1995, the NaSFiC) and the hall he was located in (only one of them) was enormous, easily three or four times the size of any Worldcon dealer's room I've ever seen. He cut a cheque for his next year's dealer's space before he packed up as he didn't want to miss the cut-off before all the spaces for the following year were sold out.

Didn't one of the Glasgow Worldcons offer a dealer's-room-only day membership? I have vague recollections it could be upconverted to a full day membership.