Blizzard considered it for Starcraft 2 and rejected it, they will however most likely implement it for Diablo 3...
The fact that over 10 million ppls play Wow show there is a market for games always online too ( even if Wow is not a single player game).
I mean the most successfull PC game of the last 5 years is online only..
The most successful game of all time is also online only. It doesn't really matter.
WoW is an MMOG, it only makes sense if it is completely online. Blizzard will not make SC2 or D3 online-only. That's just stupid.