We've seen that Twisted doesn't deal well with partially-started service hierarchies.  As far as Buildbot's concerned, it's perfectly acceptable for a scheduler to be inactive for days, while its sibling schedulers are active.  Twisted would not do well with this situation.

Also, the scheduler needs to know when to start and stop polling for an opportunity to become active.  If we overload start/stopService, then we'll have a service that's actively polling while it's stopped, and need to find some way to indicate that it should stop "for real" when the master stops.