The biggest incentive I have for having only this account is the idea of identity. I'm not one for the whole Anonymous persona thing.
If there's a way to enhance identity to a higher degree, then I'm all for it. One of the more obvious examples of this is Gamertag linking, and therefore an entire statistical background to you for your games, but beyond that there isn't much else to go on other than your posts and general community stigma.
I don't know if it's necessarily rewards that we could do with, but things that make us stand out as individuals in a community.
So I definitely like titles, avatars and the like, but they would never truly be a proper individualistic thing. It's a hard itch to scratch, this one.
Perhaps we'll see some sort of in-game screenshot or avatar creator on a higher level than emblems in Destiny. That could be cool. In fact, extend the idea of your multiplayer character to be your entire profile for Bnet too (in terms of title, avatar, etc).
I don't know, just rambling. I do think that identity is key here. If Bungie can find a way to make us all feel like special unique snowflakes, then I can only see positive things come out of it.