I used to use my mag 2x10 combo all the time with a (3 guitarist) post-hardcore type band and although i'd sometimes need to crank it a little higher than I was comfortable with (never further than about 2 o'clock), it never failed to give me a decent volume level/ overall sound - usually, but not always, having PA support.
If you've got the room in the van and the energy to lug it about, an extra cab is always better - but I'm lazy so never have