Center channels are a little off-topic, but I will give my opinion in short:
1) A well-setup 2-channel system will create a perfect phantom center for the main listening position (MLP). In music that has stuff panned into the center spot or in movies that have dialogue mixed into the center channel, there shouldn't be much to no difference between 2.1 or 3.1 systems. For the MLP that is.
2) Once you move away from the sweet spot (MLP), the effect will disappear gradually. Depending on the dimensions, with three persons in the audience the phantom center effect might not be audible for anyone but the one sitting on the MLP. Everyone else will hear the dialogue and other stuff mixed to the center coming from the speaker on their respective sides.
3) If you have a center channel, it should be matched to the other speakers so that the sound is as similar as possible. Without room correction (Audyssey etc), this would mean identical speakers or the dedicated center speaker of the very same family of speakers. Some people have reported decent success with different combinations, at least when using a room correction on their AVR's, as is should match the timbre of the speakers.
4) A great deal of audio comes through the center channel, if present. Considering that, it would be a shame if it wasn't at least up to the standards of the other speakers!
I have owned both 3.1 and 2.1 systems, all calibrated using Audyssey MultEq XT. My 3.1 system had a Wharfedale 10.2 + 10.CM combination and I was very glad with the results. That had much to do with our situation: we were two people watching TV/movies from various spots. We had a sofa and a dining table and we used both to watch videos, even at the same time. With just two speakers, I hadn't ever experienced a decent phantom center in that setting. The center channel made it all better as with it all listening positions had a clear center image in the audio! After moving to my own place, I changed to a 2.1 system. I now have two LS50's in my system and I have setup the whole room to work for the audio system. That means that no matter what I'm doing, I'm always in the center line of the speakers and I have treated my room a little. The phantom center effect makes both audio and video just as clear as my previous 3.1 system. Having experience from both worlds, I wouldn't buy a 3rd speaker for my current system/setup/living arrangement. In the remotely possible event that I'm moving together with someone, even start a family or just move to a bigger apartment where I expect to host movie/gaming parties for several people, I would then definitely get a third speaker to accommodate for several listening positions!
After writing this message I now realize that this much information was probably not needed by the people who posted about the center speakers. But since I wrote it and it might be of use to someone in the future, I'll post it anyways!