From a controllers perspective:
I deal with a lot of new pilots when I'm controlling in New York. Most if not all of them don't know what a class B airspace is, and they absolutely don't know what a mode C veil is. I have pilots flying around ZNY in the middle of the approach paths into the airports squawking standby. The problem is that they don't know the altitude restrictions of the B and when to stay below it, and when I don't see their altitudes because they are squawking standby, I have to switch radar modes to verify that the pilot is not too high and going to crash into another aircraft.
Real world, I could in most cases trust that the pilot is going to stay below the B and not worry about it - on VATSIM, I can't do that because we don't have any forced pilot training. So what is the alternative? Well, I text the pilots squawking standby and ask them to squawk C, so I at least don't have to switch radar modes to tower mode to see their altitude.