hmm, you're right nothing really came up in my searches either maybe the threads are old. I just recall last time i was able to dig up a few threads pretty easily, none really came up this time.
Basically, you get combing problems unless you perfectly phase align them in the vertical plane, which most people do not do, and then you have to sit right in that plane otherwise things suffer. It's not too different then moving off-axis from an MTM center channel. Though some people do it and claim they can't hear a difference, more power to them but it's not a good idea for critical systems. For crappy systems, HTIB type stuff, sure, but not for high-performance systems.