That's true. but having dhcp capacity for more than 255 IPs is hard (or, expensive). for a 20-30 person hackday, the organizer shouldn't be worrying about overkill network provision, and should spend that time/money worrying about good food, or something else. (imho)
for a 20-30 person hackday, it's not so much of a big problem. You wouldn't need more than a /24 for that, and you'd still be inside the * 8. But most tech events with more than 100-ish people I've been to have failed on their wifi at some point, and it's normally for this reason.
