Here's some insight into my recording process, which might give you an idea on why we choose RT's to be stereo or mono:

For me, everything starts with drums. My typical recording setup for a drum set is a mic on the kick drum, a mic on the snare, and a pair of stereo overheads. Depending on the band or genre I might add mics on individual drums or cymbals, but often they're not necessary. Overheads are panned so that the kick comes out dead centre, then I pan the snare mic so that it matches the placement of the snare in the overheads (which is typically centered with the way I set up overheads). If I have more mics than the main 4 then I'll do the same process of panning them as with the snare - the idea being to use the natural imaging of the overhead mics and use any spot mics to enhance the overheads.

But then again, a drum set is not one large instrument, it's a collection of smaller instruments put together, so if you use that logic then each individual instrument only needs one mic, with only a few exceptions. Trumpet, guitar, vocals, bass? One mic each.

More than one mic does not necessarily add to the sonic experience of that instrument - and often adds issues with things like phase, track count, EQ, and making an instrument "fit" in the context of a band mix. Typically, it's best to record most instruments in mono then pan them appropriately to give them space in the mix. Exceptions of course - if I have a horn section, that'll be panned in stereo, or if I'm recording rock guitars I'll typically double track them (record the performance twice, then pan one left and one right)

Larger instruments get special treatment, like a piano - it does add to the sonic experience to record the piano in stereo, as long as that piano is the focus of part of the music. Same with a Hammond organ - stereo mics on the Leslie tend to sound incredible, and are worth doing even if it's a background instrument. That said, if those instruments are not the lead instrument of the song, and there's a lot going on in the mix, then it's better to mix those instruments down to mono, otherwise they'll start stepping on the toes of other more important instruments.

Basically the bottom line is - if you're recording an instrument that needs to fill some space in the mix, record in stereo - although, if it's a single small instrument, you might not get much stereo out of it, in which case you'll have to create some stereo with an effect of some sort - reverb, chorus, etc...

Keep in mind when reading the above that I'm not the person who records, edits, or mixes our Realtracks (yet), I'm just going from my years of experience with recording in general. In RT's, generally drum sets, pianos, organs, and the like will always be stereo, and bass, fiddle, wind, and such instruments will be mono, but there are always exceptions.

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