You'd be surprised at how many bars and stuff still use mono systems. My take on this is having my mixer be mono-friendly for the rare chance I get to play live. Have songs that you cleanse of panning and know will sound good without it. Smart EQing and volume control are your friend.
And if phasing is an issue with DI boxes, it is most likely due to the house wiring. Often when you plug in to two DI boxes they are meant for things like two seperate DJs, microphones, or guitar amps to pump signal out to. When you throw a stereo signal into each box, they can sonically phase each other out like Dan mentioned. I've had this happen with a DJ set a while back, a lot of my mids were flat out gone, no one bothered to tell me all night either.
If you rock a stereo setup, I think it's good to make sure you have a means of forcing mono on it so the sound guys don't have to. Buy some cables, maybe a passive mono mixer or build your own and put it in an altoids can. BE NICE TO THE SOUND GUY AND TALK TO HIM. He can be the deal that makes or breaks a set, so make his job easy. Some of them are great at their job, others you are better off without them. Talk to them before you setup, you might give them time to figure out how to channel your sound out in stereo or how to help you set up in a way that won't cause technical issues.
tl;dr make sure you can rock a mono setup, always talk to the soundguys before you setup, or at least before you play