Okay, a few things here.
First, James having a powerful voice has nothing to do with this unless he's right near Petrucci. At no point will Petrucci actually hear LaBrie to any meaningful degree on stage unless he's right next to him singing loudly. No matter how powerful his voice is.
Second, yea, hearing the voice is pretty necessary for almost all bands. I know when I played in many bands, I always needed the vocals to at least some degree to know where in the song I was, it can get pretty odd without it. Obviously DT treat vocals as an afterthought to the music and the guys are probably just as familiar with the instrumental versions of their songs as they are with vocals, but it also really does cause a disconnect. You're not hearing what the audience is hearing. You're in a different world.