Demonology is, again, a debatable topic. If you calculate the cost, it takes 9 Imps, which would be 450, and one Demon costs 200 (I'm writing this from memory, there might be some inaccuracies, so don't judge too harshly). The Castle is already expensive, and then you have to buy Imps and turn them into Demons. I see the point if you have Marius.
Of course, it's more logical to use Marius. But in principle... - in random maps, you see a LOT of Imps being recruited, and as you know, they are free there :) - hence the conclusion - without exaggeration, this can be attributed to the tactics of playing as Inferno.
I like to play Heroes 3.5, where they often give a portal, but I haven't checked in the editor. In general, maybe, maybe not.
That's a misconception. You're just lucky, believe me ;)