I considered the multiple logins issue.  Yes, there could be use cases for that... we could do a class-based approach, or have that as a 
"special case".  In general, I don't think multiple logins should be a *common* use case.

Multiple simultaneous queries is possible... I think even implemented within UKIDSS.  It can serve as a workaround to bandwidth limits imposed by servers.  However, I think we should hide that functionality if we add it, since the servers probably have those per-process limits in place for a reason.