Contrary to popular belief, the center channel is not for dialogue. The center channel primary use is to anchor the audio to the CENTER of the screen for listeners who are not on the sweet spot. It just so happens that most of the dialogue is on the center of the screen.
If you listen to a properly setup 2 channel system and play any vocal tracks and sit in the sweet spot, the Voice is on the CENTER stage even if there is no CENTER speaker. Now if you sit closer to to the right speaker, all the sound will be coming from the Right Speaker. To fix this AUDIO / Video "disconnect" a CENTER channel is used to LOCK the audio to the center of the screen, regarless if the sound is from a person or a car crashing towards you.
If it was for dialogue, it would been called the "Voice" or "Dialogue" speaker :-)