demo is more than a 1 trick pony and doesn't lack any more mobility than any of the other specs... it's king of aoe, but lags behind in single target damage... but it also has a uber execution phase (soulshatter, doomguard, meta, pot, soulfire, meltface)...
to answer the OP... the most "useful" depends on the fight and raid composition... all 3 have pros and cons and are all pretty close in damage...