Why did he not fully commit the UK to a referendum, do you ask? It would have put any future, non-tory PM into an extremely difficult position. Had they not wanted the vote, they would have received huge amounts of bad publicity were they to cancel a democratic exercise.The answer is simple. 'Dave' is well aware that the tories are odds on to lose the next general election PLUS - he doesn't actually want the referendum.
Doing things this way round - he scores a great publicity victory but does NOTHING to detach us from the wiles of The Evil Empire.