I never use mercs against the AI. It's a rule I have about SP games. I never use the mercs. It kinda helps a little against the AI, but you still always win. It just makes the early game a little more challenging.
To the list of mercs I would like to keep forever I would add Renegate sage & possibly all mages of Metal Orders (the same reason - research bonus). It also allows to diversify not only magic, but troops, too. Especially if you need/want to grab some underwater provinces...