I listen to a lot of music upmixed to surround using my home theater system.
I've found various advantages (and disadvantages) to the center channel. One of the main advantages for me is it increases the sense of density to the presentation - another actual speaker in the middle of the soundstage can sound more solid than the wispier presentation of only phantom centered images. Further, throwing more boxes and drivers at the sound increases the sense of scale and heft.
Another thing I find interesting is it can increase, for me, the sense of timbral variation - that is the sense that each sound source, a trumpet, drums, piano, voice, clarinet - has a very individual character and timbre. And since I find most stereo playback (most sound reproduction in general) to homogonize the sound, this is quite welcome.
But I note, at least in my system, it makes this contribution best without using the room correction settings. So for instance, if I choose the "flat" setting, where all speakers are equalized to sound as indistinguishable as possible, the sound is more..."flat" and homogonized across the soundstage. Good for coherence and integration, but less so for the effect I'm mentioning. It's my speculation that without room correction, there's just enough idiosyncrasies in the sound coming from each speaker - due to different positions in the room etc - that they then each contribute their own slightly distinct sound. It makes for a bit more room boom, a bit less coherence, but more "suprisingness" and variation in instrumental timbre through the presentation, to my ears.
As to imaging, or maintaining the stereo imaging, I find some settings better than others, but most maintain the imaging quite well when I switch between stereo and 3 channel, or more channels. One of the best is actually a "multichannel stereo" setting on my Denon AVR, which tries to keep the general impression of stereo but spread to all the speakers, and it can sound just amazing for lots of stuff, orchestral especially.
Since my 2 channel tower speakers comprise a different sound system I just go to two channel when listening to lots of music, vinyl in particular. But I have nonetheless tried introducing my center channel too - running my L/R stereo speakers and the center through my AVR. It does add some density to the sound which is quite fun. Ultimately though I prefer the overall coherence, especially in terms of imaging and soundstagine precision, just using my two channel speakers.