My understanding is that Yusuf walked away from, if not renounced, Cat and Cat's music. He may as well be a separate person. One way to look at is that Cat has "died". He just left his body around for Yusuf to use. So while you're liking Cat's songs and trying to distance yourself from Yusuf, Yusuf is liking his holy mission and trying to distance himself from Cat's music. I wouldn't feel bad about hearing/singing Cat's stuff.

Aparently, however, Yusuf is not so committed to his new persona as to walk away from the royalties generated by Cat, and I think it's prudent to question what use those monies are put to. This is a moot point for me, as I never liked Cat's stuff well enough to spend a dime on it, but I find Yusuf to be disturbing enough to where I wouldn't want to contribute to his income.