My understanding was that mercs are barred from instances which are flagged as raid zones, and also from monster missions, but group content was ok.
However I have just seen that mercs may also be banned from some group content. Specifically the DoDH 68.5 mission 'Rage of Kelliad'. According to ZAM "Note: As of June 2009, mercenaries are still not allowed to be used in this task. They'll be suspended as you zone in to the instance."
Of course, ZAM could be out of date, but if not, can anyone tell me if this is an issue that I will come across again and again as I try to work through DoDH progression and then through instanced progression in later expansions?
Currently my group is 66 mage/66 mage/73 druid with healer/healer/tank and whilst I have been excited about catching up with old content I have never seen before (just did 68.1 and 68.2 for the first time ever), this news about 68.5 is rather depressing - especially since the boss in that instance apparently procs something which makes pets suicide, leaving my squishy group kind of stuffed!