TSS, I wouldn't be so quick to judge danimal's treatment of soundguys, especially considering his credentials and multitude of live shows that he's played everywhere. You're running very close to sounding condescending, and I know that's not your intention. I'm sure we've all thought of the ways we can accidentally tell a sound guy how to do their job and made sure we haven't insulted the sound guy, but I can also think about how many times this kind of stuff has bitten me in the ass, especially considering that I try to make it as simple for sound guys as well (I only ever send 1/4" outs, because I'm pretty certain that every place has DIs, or at least a mixer that I can send the cables to). not every sound guy is bad, so the problem has to lie somewhere else.
I think that it's more an issue of stereo signals not being common enough at venues for it to be worth memorizing how to receive a stereo signal cleanly. Venues like bars and such mainly deal with rock bands, and in most cases, there isn't a direct line-in signal ever sent, much less sent in stereo. It seems like a really simple thing to cover for! But there are a bunch of places it can go wrong (your signal to DI, DI to stage, stage to mixer, mixer settings, etc.), and chances are that your sound guy has simply forgotten about it, if only because it's not something they have to deal with. It could be something as simple as flipping the "mono" switch to "stereo", or it could be as complex as a bad house configuration that phases you out, like thebitman experienced. And then when you bring in some gear that cuts out half your track because you're only receiving the left signal, they assume it's a problem with your gear because in most cases, stereo signals aren't so hard panned as to literally play only half the song. It's basically a case of square peg, round hole, but in this case, the square peg is a rock band-oriented approach to setting you up, and the round hole is you not being a (traditional) rock band.
Even if it's a simple case of sending you into the wrong channels on a mixer, sound guys will hesitate to change their configuration up because a lot of time has been put into setting up the venue to just WORK without having to change their configuration over and over again (here I'm defining configuration as which signal goes where, etc.). Even sending an auxiliary signal like an iPod or something usually remains in the same channel, and that channel has been pre-configured to receive and broadcast something like an iPod. Having to switch up their configuration to accommodate you means a potential headache in the future when they inevitably forget that they put their DIs into a different channel or something. Not to mention that a lot of venues have multiple sound people because their main person isn't available all the time. I'm not trying to make excuses for having a simple setup, I'm just trying to reason out why this kind of thing could very easily happen.
--BEGIN UNWARRANTED ADVICE REGARDING VENUES AND SOUND PEOPLE AND YOU--
This is more for everyone else than it is for Danimal, because I'm going to assume that he knows his shit re: telling the sound guy that it's stereo, etc. The best way to prepare for this kind of thing is to make your setup as easy to RECEIVE as possible -- you can have as much complexity as you want on your end, but a sound guy will like you WAY MORE if you end up throwing the entire thing into a mixer that just outputs XLR or 1/4". I've played with a guy who literally has a van full of his gear, and has to take out three giant drawers to hold the multitude of pedals, synths, and MIDI devices he uses, but he usually ends up condensing his output to a single mixer. The most important thing to consider regarding your output is to make it so that your sound guy doesn't have to change his/her configuration too much to receive you.
The next thing is to make sure you output in a format that's common to most venues -- XLR and 1/4" are the most cited because they're the most common and easiest to throw into a DI or stage signal, which is the difference between you plugging your mixer in cleanly and you having to play next to the sound booth because there's no way to translate an RCA cable to a DI without adapters that neither of you have and the only mixer that gets you to the house is at the sound booth. It makes it easier on you too! Now instead of having to remember your 1/8" AND your 1/8" to 1/4" adapter, you just have to remember the 1/4". No venue likes it when you make it difficult for them to even receive your sound, and they would sooner have YOU gimp your own show than mess with a thing that both the venue and sound guy have labored over months, if not years.
Contingency plans are always good -- having a mono setup ready just in case is never a bad idea.
The last thing to do when dealing with any venue is to TALK TO THE SOUND GUY and DON'T BE A JERK about it, as has been mentioned above. Just a simple question of "I'm sending x, is that ok?" goes a really long way to being a good performer in general, if only because it shows you being concerned about the venue AND yourself. Then you can approach being in stereo 
--END UNWARRANTED ADVICE REGARDING VENUES AND SOUND PEOPLE AND YOU--
