I have been grouping with my cleric (75) and a Merc Cleric in groups recently. My cleric is 75 - and the merc is only 73 - but when the merc casts Promise - I find my spell blocked - so the "Overwrite and Extra Wait" time issue seems to have been fixed.
On the downside - I have a lot of healing AA's - and of course the merc has none. The merc is able to time his spells with computer precision..... so I usually never cast it if in group with a Merc. I spam heal fast heals and do other things (Gotta LOVE Divine Arbitration!).
When the mercs run outta mana - then I can still save the group. I don't actually mind them - takes a lot of pressure off me - and lets me work on nuking, curing, and other useful things.
Now if they would fix the stacking of Vie lines - I could get the dam mercs to stop overwriting my 60 minute Vie with theirs that lasts 35.... =)
Hope this helps.
Nethor