[Baalz]

Actually I think it's a balance issue rather than a thematic one. Lifelong protection, for instance, gets the forge bonus fine. The problem is that once you start stacking forge bonuses those soul contracts can get relatively cheap. If you get the Forge of the Ancients, and/or a construction bonus site, and/or innate forge bonus then its not too hard to crank out 2-3 or more of these per turn which really adds up fast. Having an auto-summoned 50 demons per turn by mid game is pretty overwhelming considering this is in addition to whatever else you can bring to bear.

That said, soul contracts are one of those things that got hit so hard with the nerf stick that they're completely unviable for competitive play. Fun when you're goofing around though.