There are sprites for them, so I'm going to assume that you can catch them. I don't see the point though, even if you do catch one no one will believe that you did it legally.
It's all about the collectibles. Everyone has an orange Charizard. Black ones are special. Some people don't care, some people care too much and will chain for hours to find one. I catch one if I find one, otherwise I'm not interested. However, I am disappointed in the shinies that look almost normal. Garchomp was a good example; they could've made it lime green or something.
Now, Pokerus, on the other hand, that's what you really want to find
My entire Black team has PKRS. I also have a Shiny Rhyperior from Platinum there. ewe
My luck with PKRS and shiny Pokemon is extrodinary. I have a shiny Pokemon in every main Pokemon game except RBY. (Shinies didn't exist back then.) I've also had a Wailord with PKRS from Ruby infecting every Pokemon with it. :3
For gen 6 you should have the abilty to connect with other people to play in the region and have double battles with every trainer as long as you have a friend. The VS Seeker NEEDS to be put back in.
Well, I believe the Vs. Seeker allows you to challenge any trainer no matter where they are (unless they indoors like a cave or forest). The PokeNav/Trainer Eyes has a limit as to how many trainers you can battle.
There was some spot in LeafGreen where I could continually face a guy with a Fearow for plenty of EXP. For some reason it gave me tons of EXP, even though they're pathetically weak.