Mercs can be good in a pinch but you can never guarentee there will will be a pool of what you need. Either the computer factions hire them out or there just aren't any (insert unit type here) available in that area.

Btw Marcrille, I play VH/M all the time and its easy enough to rotate. The trick is to either use regionals for garrisoning or have at least 2 (more is easier) stacks and have one hold the line while the other 'recharges' back at base. For defense at mid range using boats, 2 stacks are enough for constant warfare defense. Increase to 3 stacks if I need to push the boundries....and so on the further away I get. Of course it all depends on your style of gameplay. Some people just hate the micro management, I know i hated it in RTW vanilla, I would conquer most of the world with velites and town watch because it was boring to have to wait for stuff to build up.

So, in regards to the thread topic, I use regionals for garrisoning sometimes. but its nomally a very minor role and only if they are cheaper than some easily available home units.