My highest team is 77. I have several teams though and i sorely miss having a warlock if i do an instance on a team that doesnt have one. Soulstones are great to have in instances. Plus i sometimes get some weird error where one of my team members cannot use a mage portal created by a mage in my own group. When is happens i can just summon the last one there. Plus locks in my groups always seem to do better dps than the mages.

Finally, i think locks have some kind of crit or damage buff they can cast on other people at higher levels. I would switch out a mage for the lock.