It's not that bad if your minion generator is a creature, rather than an object. It didn't take long for my players to figure out that it was the undead leader type who was summoning the dead from the surrounding graves, at which point focus fire dropped him like a rock.
My experience was of a trap-object minion spawner, the DM arbitrarily forbade any solution other than solving the complex logic puzzle, and it just ground on for 2 hours or so. It might have been ok in a computer game; as 2/5 of a 5-hour game session it was terrible. Especially as, after a nice 30-minute fight, we spent the remaining 2.5 hours grinding on a solo zombie dragon. Apparently we were supposed to use some stuff that was lying around. My INT 8 dwarf barbarian was completely unsuited for either encounter. Possibly the least enjoyable RPG session I've ever played, that I didn't actually walk out of.