to address what joe is talking about...
the matrixes that matt and i used to do were done \'on the fly\'... we had a mixer that brought in the aud (on-stage) mics and sbd feed together, and they were mixed as one before they hit the DAT. this was a very simple way to do a matrix.
unfortunately, there\'s other factors involved here.. you can\'t do the type of matrix i talk about above with mics out in the audience, because there is a delay between what the mics hear, and what\'s coming out of the board. this is the reason we always had mics on stage. by doing this, you lose your audience recording, so if the board feed sucks, and you end up with a shitty matrix, if no one else is taping, you\'re stuck without an audience recording.
as time has passed and i\'ve moved onto computer recording, i run a matrix at pretty much every show i record. the difference is, i use actual audience mics rather than having them on stage, so i always have 2 recordings. the sbd/aud delay is not an issue here, because i\'m recording 2 separate files. afterwards, i can line them up any way i want.
unfortunately, doing a post-mix of a matrix is a timely process, because i\'m not just gonna set it and forget it.. i\'m gonna sit and listen to an entire show, and adjust sbd/aud levels accordingly, then go back and listen again to make sure i\'m happy with it.
another factor, and a reason why you don\'t see more matrixes on disclogic, etc, is that about 80% of the board recordings i get have clipping throughout the entire show, that i have zero control over. this makes the show unlistenable, and is why you\'ll only see aud recordings of these shows.. this goes back to what josh said about different boards - if i get a board where i can\'t control the gain of the output for my recording, it\'s most likely not gonna be usable..