Why not? You paid for them, so do whatever you want. If Necropolis recruits everything, they won't have much money left. Besides, creatures of level 2 cost a lot, so they should only recruit level 1 creatures (I tested: 84 skeletons cost 5000 gold, 52 Greymarch Centaurs cost 4600, 38 Grem cost 1500, 71 Phantoms cost 1500, but there aren't that many fairies and imps; for example, 37 Greymarch Dwarves cost 5500), and not buy the native army of Necropolis. Also, the chance of getting the Diplomacy skill is quite low for all heroes, except for priests, and the chance of heroes with Diplomacy appearing in the taverns of Inferno and Necropolis is lower than in the light castles.
Maybe someone knows the mechanism for calculating the cost of joining creatures, but I think my empirical data shows that Diplomacy is not that cheap, and there aren't that many level 1 creatures with low all value.
P.S. Not everyone joins for money; I often get creatures to join for free. It's like with morale; it might work on all stacks with +1, or it might not work at all during a battle with +3)).
