I have some experience doing this kinda thing... Just tracking the output of the board will give you a bad mix. As explained by MONTE. Actually, it can only give you good results in huge situation where the sound that comes from the amps and drums on stage is nothing compared to the speaker outs.
The next way to go is making a seperate mix using an aux (or 2 for stereo). With decent cans or with a soundcheck (track-listen-adjust-track-listen-adjust...) before the show you can get pretty ok results. Done this a few times.
And the next step up is using a multitracker, but unless you have a huge amount of tracks (one track/mic, only levels to keep an eye on), you will still need the cans to make decent submixes. I did this on a show with 7 bands a while ago. Needs ALOT of preparation, is 2 times as much work as just mixing live (keep an eye on the levels, make 4 submixes, rewiring the inputs from aux and direct outs...) Don't try this if you haven't got a live mixer that knows the band and knows how to get a good mix in a short time. And believe me, unless he's VERY motivated, you won't get the guy to do it more than once every half year... Adding a room mic here is a very good idea, if you have the tracks...
And there's another alternative, that I haven't tried. Just track a stereo roommix, preferebly from the position where you mix.