The purpose of difficulty is to keep the block time around 10 minutes; if you use it to perform some other function then you'd end up with unpredictable block times. I think setting the block reward to a function of difficulty would stabilize a cryptocurrency[1], but that very stability would probably lead to failure if the resulting exchange rate and difficulty are very low.

[1] https://blog.ethereum.org/2014/11/11/search-stable-cryptocur...