Candidates like McCain, Christie, Bush, etc. serve two purposes for liberals. First, as you say, there's the hope that conservatives sit out the election and hand it to the Democrat. But even if they don't, a liberal Republican will get elected, somebody who may not give the Left 100% of what it wants but will at least give them 80-90% behind all the rhetoric.
So having Jeb on the ticket is a win-win scenario for the Left.