Surprisingly, my follow up
is not "Absolutely Nothing"!*
However, like most audio questions, it depends...
I have been using Paradigm Signature S-2's for my mains with a phantom center (as in no center) for my HT.
I always wondered if I was missing out on anything by not having a center. Dialog is clear and I've never really felt an issue. The cost of a Paradigm Signature Series center is beyond my desire to find out.
About 6 months ago, I picked up a pair of Energy RC-10's from Fry's when they were on sale for $220. They are great speakers at $220 and when Fry's put them (and the RC-LCR center) back on sale, I quickly set up for A-B comparison to see how close their timbre was to that of the Sig-2's. It was close enough that buying a RC-LCR for $220 (with a 30 day return policy) was worth trying.
My interest was two fold:
1) Did the RC-LCR mate acceptably with my Sig-2's and
2) Did having a center offer any significant benefit.
Question 1 is TBD. I just finished watching Star Trek Into Darkness (thanks Zieglj, for the suggestion) and it did nothing to catch my attention as sounding wrong, but I am too ADD to watch a good movie and remember I am trying to listen carefully! There were places where the center seemed to call attention to itself. I am not sure if that is a fault of the speaker or simply a result of my never having a center before. I am waiting for my copy of "The Fugitive" to arrive ($7 and change at Amazon for BluRay). Wayne Pflughaupt suggested a specific scene for evaluating the sound panning from main to center. I will follow up on this thread in a few days after I get the chance to listen to this section.
For Question 2, I set up level matched to A-B between the pair of RC-10's and the Sig-2's plus RC-LCR.
Based on this experience I will add a center (whether it is RC-LCR, or not).
This is, of course, purely subjective opinion, but from my observations, I feel:
1) A center would be of little benefit if your mains are close to and equally spaced from the TV screen. I experienced no significant increase (nor decrease) in clarity of sound.
2) If you have a setup like mine, you would benefit substantially from a center! (see photo below). My listening position is about 12 feet back and perpendicular to 10" in from the right edge of the 60" TV.
It is pretty obvious, but when I run just the mains, the sound is centered right of the screen. Adding a center does a great job of puling the sound back to the screen. In some ways, the LCR sounded worse because it narrowed the sound-stage, but for the purposes of HT, it was beneficial.
Again, I believe if the right speakers and room were symmetric with the left (equally close in with a side wall to match) adding a center would have done very little.
My observations are probably a no-brainer for many of you, but I figure there are plenty who are uncertain about the wisdom of spending money on a center and I hope to help them decide.
As said above this is based on my specific subjective experience. I welcome any comments to help define the potential benefits of a center.
(7.5' left of door, 4' doorway, 4' right of door)
Please excuse the wires and chaos, I don't have in-wall wiring for the extra fronts and subs, or 2nd receiver to TV!
* Reference to classic Edwin Starr hit