I had the opportunity to listen to recordings made on matched schoeps a while back and I remember the recordings sounding beautiful, but it was so long ago I don't remember what the recordings were of. I do remember the sound of recordings I made 5 years ago of a cello, flute and guitar being performed in a large 6 foot tall, 50 or so foot long colvert that went under a back country road near the Sandia Mountains just north of Albuquerque, NM.
I set up two AKG C-451B condensers (Serial #: 22793 & 22869); unmatched) approximately 5 feet into the colvert, the cellist about 15 feet deeper into the colvert, and the flautist (or Flutist; which ever you prefer)/guitarist another 20 feet deeper (using distance to create a natural mix presence). The stereo image was pretty well intact and balanced (both image wise and sonically). As you all know microphones capture everything, especially the things our minds tend to neglect when listening bare. At first I thought my levels were overloaded because of a strange sound that made me think the recording was distorting, then I realized the sound was a result of dry hairs on the cellist's bow scraping harshly along the steel strings of the cello. I asked the cellist to put more rosin on his bow. Unfortunately he forgot to pack his rosin that morning.
The recordings were a mixture of some very beautiful soft passages played in a largo and andante tempo, then moving quickly into moments of allegro and vivace, which caused the scratching of the bow; thus killing the moment for me. I don't think a matched pair would have made any difference in helping this session. Fortunately it was just for fun, not money; and the recordings were for private use of the cellist and his friend, the flautist/guitarist.
If I was hired to record an ensemble for real I would probably set up a matched pair of schoeps or another microphone similar in quality just for the heck of it. But as Chuck Russom mentioned, I don't think it would matter much with todays technology. I'd also consider setting up a 5.1 array just to give the ensemble the option to be ready for a surround release...and charge a bit more.
If your purpose for creating a stereo pair is solely for ambience recording, then a matched pair is not necessary. Tim Prebble makes a great point above, and when you watch his videos and listen to his recordings, you see that his approach is simple, using varied microphones, yet his work is stunning.