You hardly ever pan main vocals anyway, so you're normally using your stereo pair to produce a mono sound. In other words, both speakers are producing the same sound at the same time. Since hardly anyone in the audience is an equal distance from both speakers, they'll hear a messy comb-filtered version of the main vocals, rather than them cleanly.
You can make a very good argument for not bothering with stereo at all for live sound (except for effect stuff, but then we're back into separate-pa-for-each-instrument grounds...). The trouble with that is how much harder that is to rig. If you're stacking stuff on the ground at all, then you can't really stack center stage, you need to stack to the sides. (Or behind the stage, with all the feedback issues that involves). If you do fly your PA, then you still need legs, usually. And a big PA is stinking heavy, and flying it near the legs is a lot easier than near the middle, due to lengths of truss, loading points, etc.
I've never tried it personally, but people who's ears I trust all say that besides all of that, just using a secondary sound system for vocals still makes them come through a lot clearer and everything sound better.
One possible reason for that is that a speaker is essentially just a paper cone with an electromagnet on it. There are physical limits on what that one bit of paper can do in terms of vibrations. We like to pretend the world is a lot more digital and measurable than it really is. By separating out the vocals from the other instruments, you allow the speakers to only move when the vocalist is singing. I can see why it could be cleaner. I believe the Chilli Peppers may be doing 2 PAs at the moment splitting off the vocals to their own mix...
You'll still always need a good sound guy, and you'll still need somehow to control the (vocals:instruments) ratio in the audience.
Ideally, bands would think of the sound guy as a member of the band. Kind of like the conductor of an orchestra is a vital member of the orchestra. A good sound guy is very similar. A bad sound guy has all of the same control, but none of the feeling.