Though I love James's performance, guest vocalists would've made The Astonishing a fantastic album instead of a great (disc 1) to good (disc 2) one.
The reason why some masterpieces like, say, Human Equation and Jesus Christ Superstar (and to lesser extent Tommy) work so well (on top of being goodly written music) is due to the interacting performances of a distinct cast (with each voice fitting the role, offcourse). A critique TA often gets is that it is too monotonous (many similar piano ballads, for example) or drags. And I agree, while I generally like the whole album, it is not something I can listen to in one go. Guest vocalists, in my opinion, would've eliminated this completely.
To me, this is a bit frustrating to be honest. I really feel like there is something greater there and they went all out with Campbell yet failed to utilize the full potential of the album by sticking with James only who, again, did a fantastic job. But having one voice just doesn't align very well with their rock opera ambitions.
(needless to say, I am glad with having that cover of Act of Faythe and having the singer of Halestorm on Our New World, though the latter doesn't sound anything like I imagined Faythe).