I do like what an X/Y or M/S pair does between the wide omnis, as long as you are into tweaking the Mid/Side ratio of either for best effect in addition to playing with the level balance between the center pair and the omnis. But that's sort of gravy on top of the main course of the omnis + single center mic. Just putting any mic between the two omnis goes a long way. Bringing up some of the rear-facing mic in your mix effectively does something similar to what adjusting the M/S ratio of a forward facing center pair does- it opens up the sort of "hard-panned SBD sounding" center somewhat and blends it with the wide open and deep ambient base which the omnis are providing.
I was experimenting with using a M/S pair in the center between omnis just last weekend outdoors, and to do so I gave up my usual rear-facing mic channel to be able to run the Side figure-8. However, I don't think I'll move to doing that regularly, partly because I also was running a sideways facing pair of near-spaced supercards which also do something sort of similar in combination with the single center mic as turning it into a M/S or X/Y (m/s adjusted X/Y) pair does. I just I like the spaced sideways-facing supercard pair a bit better in a stereo mix, and a whole lot better with discrete multtchannel surround playback do to improved L/C/R channel separation.
Here's that arrangement as a full 8 channel array with both center forward and backwards facing M/S pairs hidden inside the big Shure windscreens, the omnis are cropped off out of frame, the mics at the edge are the sideways-facing supercards. Last weekend's setup looked identical to this, except the rear facing M/S pair wasn't hooked up so I only needed to record 6 channels. A good test, but I missed having that rear facing mic!
But if I had a wall reflection directly behind me or a noisy beer tent something, I'd not use the rear facing mic and switch to an X/Y or M/S pair in the center instead.
Hope that helps more than it confuses!
Link to the thread documenting the evolution of these crazy setups- http://taperssection.com/index.php?topic=96009.msg1279052#msg1279052