This begs the question of what is appropriate. Is a super tight focused Mid best, or a nice wide-open one? I think the answer probably depends on what is going on with the rest of the array.
Thinking this through, potential rambling ahead..
I originally went with Mid/Side for both practical and theoretical reasons. On the practical front, I came from originally using an omni in the center, and wanted to try using a directional microphone in the center to increase focus on the pickup of direct sound from the stage and PA in that channel and exclusion of sound arriving from other directions (that already being well handled by the wide L/R omnis). I found that change to be useful and decided I wanted to maximize that as much as possible so I started using a supercardioid instead of a cardioid in that position. The idea being to get that center channel sounding as sounboard-like, as clear and dry as possible. Okay good, but maybe too tight with that center image not blending as smoothly as I'd like with the omnis.. so I add the Side fig-8, figuring I can introduce as much of that as I like, which not only widens the center contribution to blend with the omnis, but does so in a stereophonic way. That worked well, and adding the Side fig-8 didn't require altering the previous base-line configuration of the single center mic - I could just mute the fig-8 to return to that configuration for comparison. Good so far.
I speculated about substituting X/Y.. probably best as PAS with M/S readjustment, perhaps even better than M/S in the middle because it's native PAS. Seems to work well.. The questions remains not fully answered.
But I then added the near-spaced supercardioid pair and some new very interesting things happened and options opened up. In some ways that pair was more up-front and direct sounding, with good presence. Which leads to the question- should that pair be the primary PAS direct-sound contributor instead of the center? ..in which case, would the center Mid/Side pair then be better using a wider Mid? ..displaced from its previous primary role of providing "direct clarity" and taking up instead more of a "fill and glue everything together across the middle" type of role? I don't know the answer to this yet.
Also, I really like the 3-mic supercardioid triplet in the center formed by the addition of the near-spaced pair (fig-8 muted). It's somewhat sharper and clearer than Mid/Side own its own in the middle. I feel the center mic needs to remain supercardioid for this combination to work best. Hmmm. For a while with both the triplet and M/S center in place I thought I had extra redundancy there, and the answer might end up being either the coincident center or the triplet center.. maybe triplet for playback with a center channel speaker and Mid/Side for 2-ch stereo. But since then I've found what I like most is first getting things balanced in 2-ch stereo using the triplet, then adding some amount of fig-8 Side as "special sauce". In that way it has become akin to the rear-facing mics for me. How much to use varies by recording, by venue, sometimes song by song. I've considered automating it's level in the mix. I just know highly I value having both. At least I've determined the hierarchical importance of each channel addition and know which I'd give up first if necessary verses which I'd keep, which has remained consistent recording to recording, so that keeps me sane.
Could freely adjusting the Mid/Side ratio of a PAS X/Y pair in combination with the addition of the near-spaced pair work in the same "special sauce" kind of way as how I'm using and thinking about the Side channel? I can certainly see that possibility.