The only mercs that I ever get "attached to" (try hard to keep safe) are Cretan Archers because they are excellent archers and Sarmation mercs because they are just so cool. Almost all other mercs are used to gaurd my flanks.

Barbarian mercs I use to fill the role of cavalry when I have none. They can run around to the rear of the enemy line and pin them against your line. Their weight of numbers helps greatly.

I also use them to quell riots.