Please imagine picking up a piano trio (furinstance) with violin, cello, and piano with a stereo pair in a concert hall with noise from the heating system in the back of the hall...you're going to have to lean on a dishy reverb...
The violin is stereo left of the pianist (so he can visually communicate with both the pianist and the cellist), and the cellist is almost nestled to the right of the bentside of the piano.
The piano fills the hall, (no doubt), and you need to balance the two string instruments with it.
Try that with a great pair of omnis...muddy, with too much piano and too much piano in the room (remember, you're picking up two whole spheres of sound with them).
Try that with a pair of large diameter directional condensers like the U87...ragged off-axis response on the highs, and too much LF information to maintain clarity.
Try that with a pair of small diameter directional condensers like the KM84...things are tidying up nicely now, but how about that low end rolling off and the piano is still too prevalent in the mix?
Try that with an ORTF (or reasonable semi-coincident equivalent) pair of U89's or TLM170's set to hypercard? (in the right place...there's the art to all of this)
The off-axis response is less ragged and peaky on the high end, so it's much more kind to the strings, and the piano, especially if it's voiced on the bright side.
The low end response doesn't roll off as quickly as a SD cardioid condenser, and with the hypercard pattern with the on-axis pickup favouring the strings, you're partially rejecting the piano (balance, anyone?), and you've got something decent to send to your reverb.
A couple of dips in the on-axis frequency response of the microphones that don't make the strings too screechy helps, too. The audience goes nuts with approval and that WAY-off axis stuff actually sounds like something in real life...not overly-coloured.
Yes...the electronics are about as grey and lifeless as they can be, but ask many classical recordists, and the K89 Neumann mics (U89 and TLM170) will pull the job off with minimal EQ (or the ideal...NO EQ), and hell, you might even have to put a couple of SD cards on the piano because it's relatively further away and needs presence.
A fun project...K89 capsule, and minimal-phase-shift/minimal feedback (same thing?)/low-noise electronics (I'll take transistors or tubes), and I'd have a mic I'd be happy to use when the acoustics are less than ideal and I can't use a great pair of omnis.