The nerf was to prevent Monster Nest abuse with Nekkers. Nekkers were breedable before they were reworked and probably shouldn't have kept that trait afterwards.But then there's only 2 cards that i know of which are breedable now....and one of them in incredibly situational. Harpy eggs and Arachas. I can't think of any others...
What a stupid nerf.
Other breedable cards are the Golem, Fire Elemental, and Earth Elemental spawns. The Woodland Spirit and Caranthir spawns may be breedable as well. Ge'els is getting reworked anyways so it's going to be a while before we see what the state of Monster Breeding is going to be.