Good thread! Now, my two cents. I think it depends on L/R speakers imaging like previously mentioned by others. And I agree with the comment about placement
I have been wanting to debate the necessity of center channel speakers because I spent a good portion of my HT investment on L/R speakers ( seeking good musicality) and it seems to have payed off in that I don't seem to need a center speaker. I was ready to spend a grand if need be which is the price for the matching speaker. But I have held off so far because the illusion is perfectly placed mouthed dialogue on my system. Here is the thing, perhaps, my speakers are reasonably tall 39", and they are only 7 feet apart. I think that once you get more feet of seperation in the equation then you will indeed want a center speaker for dialogue placement. I like my wide explosions. And I like listening to music in Dolby II Music mode and (might be all wet here) but here goes--If I had a center channel speaker connected it would be sounding and this does interfere with stereo imaging. Lastly, I believe headroom is possibly better. Check out bench test results for AV receivers. Each channel is not entirely discrete i.e. the more channels driven the less watts per channel made available by the receiver's transformer. Am I completely off base here? Jump! (Insert (goofy sense of humor) visual of a football tackle pile on here).