Every single turn that I can hire merc units I like (sarmations, cretans, and war elephants) I hire them and place them in a "replacement depot", most probably a city near the front. Everytime my merc units serving in my armies take casualties, I merge one of these new merc units with it to "retrain". I've been able to do this with full units, so I don't know maybe they changed this in the patch because I'm still playing vanilla RTW.
Playing as Carthage, my main battle line was also spanish mercs until I could tech up to at least libyan spearmen. Even then I liked them for their javelins. So, for some factions, hiring mercs at specific points of the game is useful.
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