This theory is based on the fact that every magic system we have encountered so far can stand on its own. While some magic systems such as the metallic arts interact easily with each other (which can be explained by them being on the same planet) most other systems require various hacks to get them to interact.
My theory is that the powers stealing aspect of hemalurgy is actually such a hack. This may sound weird because of the way we have seen hemalurgy so far, where it is mostly used to steal powers. We need to remember however that two of the three hemalurgic creations of TLR we have seen so far didn't have anything to do with allomatic or feruchemical powers. It also would have been weird that a magic system would be created with the only purpose of interacting with other magic systems.
My conclusion is that the original purpose of hemalurgy is the transformation of creatures and to give yourself limited boosts, such as a bit more strength or intelligence. The stealing of powers is just a (perhaps intended) side effect.
