Yes ... as he says, the ECM8000 is an omni mic. If you used it in X-Y, you wouldn't get a stereo recording (or not much of one), you'd just get two tracks of virtually the same thing.
If you were to use that mic for stereo recording, you'd want to use a spaced omni arrangement, perhaps with a baffle between them. The problem with this approach, at least on a guitar, is that there's some tendency to get odd phase cancellations. At a particular position and with some distance, you might get something good though. Of course, with distance (and an omni), you're really going to pick up a lot of room sound, which is fine if your room sounds "good" (or at least sounds like what you're going for), and not so great if it doesn't (as another poster mentions above).