1) Problem during launch. There's been one successful use of a launch escape system in rocketry history, and such a system has never been developed for a shuttle type craft.
2) Problem after entering orbit. Cutting your orbital flight short isn't going to change the fact that you still have to survive re-entry stresses and have a working vertical landing system.
I'm not sure if you're aware, but this is a feature of the latest version of SpaceX's capsules. I'm sure it would exist for their larger vehicles.
edit: although I'm just not sure what you'd do during an abort with all that fuel that larger vehicle holds...
The fact that the BFR ship serves as both second stage and capsule combined, whereas the Dragon 2 is just a capsule with a separate second stage to put it into orbit, makes the problem a lot harder. I bet that the system for the BFR will consist of "don't explode."
Oh, that's interesting. What makes you say so?
The reason seems to be a combination of NASA not being interested in propulsive landing (and they're funding most of Dragon 2) and deciding to go all-out on BFR instead of refining Dragon 2 further.