Hi, hollywood_steve--
Coincident recording methods have undeniable advantages--image stability and mono compatibility are the two biggies, and simplicity is a very good thing, too. But to get those goodies when the hall and/or the arrangement of performers is less than ideal, X/Y recording always forces you to give up something else that you want in the recording. If that "something else" is a decent overall balance among the instruments or voices without favoring the front/center performers too strongly, it's time to start making intelligent compromises with the recording method.
I was talking about one basic problem of recording a large ensemble that occupies a wide stage. In that situation, a pair of centrally-placed coincident microphones may be literally ten times closer to the players nearest the conductor's position than they are to the farthest players. This makes the front and center players seem louder and clearer than all the other players. The extent of this effect will depend on the actual distances involved and the characteristics of the hall. Listeners in the audience always hear some amount of this effect, too, so it isn't something that absolutely must be overcome completely. (That's fortunate, since there would be no practical way to do so.) Rather, it's a matter of degree--of not having too much of any one bad thing, so to speak.
If the microphones are moved closer to the ensemble, the contrast in clarity and loudness due to the differing pickup distances will increase, while if the mikes are moved farther back and/or up, it will all "even out" more. But varying the overall distance also affects two of the other primary factors that we have to care about: the balance of direct vs. reverberant sound being picked up and the stereo image width. No law of the universe guarantees us that the optimal distance for any of these three "matters of degree" will match the other two at all. The contrary is more often true, in my experience.
The worst case is that there are particular situations in which the microphones must be placed relatively close to an ensemble--the hall is too reverberant or too noisy; the character of the hall sound is not so pleasant; the program material requires extra clarity; you have no access to places in the hall from which to "fly" the microphones; your favorite brand of oxygen-free microphone cable is so expensive that you can only afford 18 inches of it. In those cases, spaced microphones (especially if they're relatively widely spaced--the style often favored in the U.S.) will give you some extra "wiggle room" to choose a miking distance without favoring the front and center performers quite as much as a central X/Y pair would do.
--best regards