I don't know. The voice is a single voice why would it be in stereo (maybe effects not the voice). I'm also not a huge fan of doing wide pans on the drums, even for recording, nevermind live. When the drummer plays a kit they aren't spread from one wall to the other, it's basically a single instrument. Depending on how big the kit is it maybe spans 5 or 6 feet ... My guitar is probably a 3 and a half to 4 feet (funny I have never measured it) but I don't pan the low frets to the left and the higher frets to the right, so why do we do this with drums?
I'm also not a big fan of stereo for live performances, why should people on the left side of the venue hear things differently than people on the right. Stereo can definitely assist with spacial reference... for someone who is sitting in the sweet spot. Which works for the home stereo, or for headphones and ipods. For live sound not so much.... not a fan. Even in the car I have to mess with the balance to make is sound right to me since the drivers seat is on the left side of the car, I hear left speakers more than right if I just left the balance control in the middle.
So can you create a more interesting monitor mix for IEMs utilizing stereo... sure. But that's a luxury that to me would just take too much time and isn't necessary at all. I think there are many of us who have no problem singing and picking out our vocals whether using IEMs or plain old floor wedges. Been doing it for years and years and managed.
SO ... while I don't want to take away from those who want to take the time to set up stereo IEM mixes, fill your boots if you want to, but to me it's just a little over the top, I would never bother, even if I had a setup that could handle it.
Monitoring is a tool that allows the player to hear what he needs to be able to play his parts. It's not supposed to be cool or sound like a CD.
In fact I've been using less and less monitoring as years go by...
I now use only Me, snare (on larger stages) and a tiny bit of vocals (just enough to understand the lyrics in case I get lost).
That's it. I can get the rest from the PA and back line of the other musicians!
Every body asks "why the snare?" because I'm a guitar player and my place is over the snare. I should leave the kick to the bassist!
And I don't need the kick on monitors to know were it is! I can feel it vibrating under my feet with all the subs.
By the way, I avoid in-ears every time I can