I'm not at all saying that Dream Theater isn't "allowed" to have multiple vocalists because Ayreon does. What I am saying is that the album - in my opinion of course - would not be as good as it is had they done so. I don't think that by not having multiple singers that they didn't go "all the way." They certainly did, and more singers aren't required to make the concept complete.
The Ayreon comment - had they used those particular vocalists, well, they'd still be "allowed" to do it, but it would certainly be reminiscent of Ayreon, especially with Anneke. I've seen her live, and I appreciate what she does, but James nailed that performance (studio and live), and it's better with him doing it. I've also seen Devin, and I like him a lot, but again - same thing. I'll take James as Lord Nafaryus any day.
If this makes me a Dream Theater purist, so be it. I think it just makes me a fan of what James LaBrie did on that album. In my opinion, he put everything into it and turned in the performance of a lifetime. It's my favorite vocal performance period, and I wouldn't trade a second of it to have anyone else on that album - not even other favorite singers of mine, certainly not Anneke using too much vibrato while singing A Life Left Behind.
If I ever for a second doubted the above feelings, any doubt was erased when they released that version with Lzzy Hale. Don't mind them doing it as it's not on the album, but I wasn't a fan of her voice on it at all.