It was only a phase...
I remember phase issues after summing to mono was quite a big problem in the past. It's not such a big problem now, as there aren't many mono playback devices left in use.
However, to me, it's still pretty much a must to fix them, as I can hear most larger phase issues on my headphones, and I also see them in various displays I use to monitor things like stereo width as I mix, so I like to get it clean as soon as they show up (usually due to extreme effects). I hate it when I hear different kinds of shifting phase issues on TV, too. To me, it's quite unprofessional.
My limited knowledge on field-recording:
Microphones aren't really my thing, though, so I don't know if field recordists worry about phase issues at recording time that much. But intuitively, I think it would be worse for things relatively close to the mics, of different distances from each mic, of lower frequency, and with increasing the distance between the mics. But I'm guessing there (They may be some fading uni lecture memories).
I'd just listen live and on playback, with decent monitoring headphones, flip between mixing the mics (Mono) and separating them to L/R and see if anything becomes apparent and if so, try varying the technique.
Why is X/Y stereo micing better with regard to phase issues?
Let's start at the beginning - The brain learns the differences in transferred sound arriving at the head from different directions and uses this information to estimate the direction and distance of sounds, as well as environmental features. A big part of this is relative differences between the left and right auditory systems. One of these relative differences is the intensity of the sound, another is the time the sound arrives. Naturally, when a sound reaches two receivers at different times, the two signals are going to be out of phase. This is fine - until you need to add them together.
The AB microphone technique utilises this natural delay, as well as sound level, to create a more spacey, more "realistic" stereo image. But the downside is that when summed to mono, the same sound in both signals but at a different phase can manifest as comb filtering.
The X/Y microphone technique attempts to keep the stereo image while, ideally, removing the delay by positioning the diaphragms as close to the same position as possible. This means the sound will arrive at both mics at virtually the same time. So for X/Y, the stereo image is simulated using only sound level differences in the microphone's angled pickup patterns.
So that's why you're not likely to have phase issues with X/Y - the sound arrives at both mics at virtually the same time, meaning they're in phase. There are other techniques in the first link below, but that's pretty much as far as my knowledge on microphone technique goes.
Stereo Recording Techniques and Setups (DPA)
Head Related Transfer Function (Wikipedia)
Comb Filter (Wikipedia)