I do the reverse for dailies since autofollow breaks a lot in Icecrown. I mark my main so I can see on the small screens if I'm still on track. Often on the main screen I'll lose visual of the slaves even tho they are still following, so can never be sure without looking at the other windows. Also, I think one of the multibox software programs used by a lot of peeps here has a message system that'll notify the main if follow breaks, haven't used that tho.

But I also agree that fights in dark places or blocking objects raid marking slaves can be very helpful.