I agree with people saying the album would've been better with more vocalists. Or at the very least a female one. JLB is great on the album, but one voice can only do so much for a concept double that strives to be something like, say, Jesus Christ Superstar.
And in regards to the tour, if bands like Kamelot can do a full tour with multiple guest musicians, so can Dream Theater.
That said, I do really like The Astonishing but think there is a lot of untapped potential.