The Gray Prince in Oblivion just looked pretty much like a pale Orc to me. :confused:
I would play one simply because it would make for an interesting backstory.
That's really all he was. His mother was an orc so he's an orc. He just has some vampire-like characteristics, like pale skin and incredible strength. He's still orc through and through.
The Bretons are a special case. They're not really half-elves, they're still fully human. They just, as a race, have a lot of Merish characteristics in their gene pool (or whatever the TES equivalent is), to both good and ill. They have a lot of magic resistance and naturally deep Magicka reserves and a slightly longer life span than most humans, but they age terribly, always becoming very feeble in their final years. Not sure what the source on that last bit about aging is, but I'm sure I heard it somewhere.