>I agree with those suggesting some sort of center channel. Just because a system images great with music, don't expect it to make sound appear to be coming from the actors on the screen.
A system which images well for music does even better for film. You perceive sound to be coming from peoples' lips even when they're not centered on the stereo image.
An identical center channel, with identical spacing to room boundaries (screen-wall, floor, ceiling)and no nearby objects to reflect the sound (like a television or entertainment center) will sound the same as the mains. If you don't have such a setup (which requires an acoustically transparent screen and equalization to compensate for it if all three speakers aren't behind the screen) it will sound different.
At which point, you're choosing between the lesser of two evils. With severe off-axis seating or asymetric left+right speaker placement you'd probably want a center. If you're not in that situation and can't get a sufficiently similar center channel you problem don't want one. In between it's personal preference.
I've used a pair of Linkwitz Orions for home theater + music for the last 3+ years and not yet been motivated to build a matching center channel.
In my current living arrangement, I've toed the speakers in so they cross in front of the reclining love seat I have to sit-in. The image is no less centered in the left or right seat than when we used the TV's built-in speakers when you aren't watching anything.
Previously, I sat 11' off the screen wall with the speakers 4' out, toed in to form an 8' equilateral triangle with the center seat. The image shifted a foot or two when you moved to one end of a 7' couch, although you wouldn't notice when something was being projected.