I know, I proposed it myself on June 22, 2007, on the 18th page of the Wish-List for Dominions 4 thread
Thus my above thanks to Foodstamp, and my sincere hope that someone will finally do something with it, after a year and a half
"The merc thing isn't bad, although the way I'd do it is give the commander units the Merc ability, which makes them freespawners for whatever their particular specialty is-elephants, icthys, villains, pirates, whatever, and then you hire the mercenary captain from the merc pool, and you get the captain and whatever starting troops, and the captain freespawns for whoever's paying the bills.
The longer you pay, the more benefit you get."
I was, myself, inspired by Chrispedersen, who proposed that mercs be able to recruit, to "get a little strength back".
So, ultimately, this is an old idea that Chrispedersen apparently forgot he'd had.