The first thing that came to mind was, what if you want to be a builder when you join?
Theoretically there could be a variable that the client sends telling the server what default class they wanted to spawn as, but that seems like more trouble than it's worth.
Alternately the client could be set to automatically send a spawn request of the desired type when joining a team, but it doesn't take long to select a class form the list anyway, so I don't see how that would really help.