Yeah, what Beth said goes for me, too. What is Etrigan without Jason Blood? Jason's not just Etrigan's "secret identity." He'd be a compelling enough character to lead a book on his own (he's even got the distinctive visual attribute necessary for a comic book lead in that silver streak of hair); the fact that he swap out with Etrigan is just gravy.
Well, not gravy for Jason, obviously, having that demon deep inside him the way he does.