WE share all the time at WDW. Why waste food? It's never been an issue. WE've never done the dining plan. Often we'll order an appetizer or two and one entree. Sometimes, we might each order entrees, but one dessert. It's never been an issue anywhere. We've also ordered one entree with an extra salad or veggie or soup. At WDW, no matter what, our family often shares everythign anyhow. Who needs to eat a full order of fries? A whole box of popcorn? or an entire brownie?
I also order the kiddie size ice cream at soft serve places, because that's all the ice cream I can eat. If they have to charge me for the next size up, so be it. Usually they don't though, and I'm less likely to return if they do.
I wouldn't dine somewhere where it was an issue. The obvious exception to this would be sharing an all you can eat skillet (or buffet). Still, even with that, they always bring a complete skillet. I don't eat bacon, so it just gets thrown out. I would tend to skip the skillet unless someone else at the table was getting it too, or I'd tell the server to skip the bacon. That's also a problem I have with LTT. I don't eat the ham, so I hate for them to bring it to my table. It's not like you can take a doggie bag from there.
I also can't eat and drink at the same time... I have to do it like a half hour apart. So, to remedy this, I try to pick of DF's plate generally. And, he orders one heck of a big meal! 
