"Even a vote for Nick Clegg is better than not voting" says John Rentoul.
John Rentoul. "Independent"
Refusing to vote, stems not from voter apathy, but from the cynicism and hypocrisy of politicians of all parties.
A vote, even for Clegg (perhaps the most hypocritical of all the motley collection on offer) as you suggest, indicates an endorsement of the party or the policies proposed.
The "party manifesto" has only ever been (with the possible exception of the 1945 Labour Party manifesto), an inducement to place your "X" in a particular box on the ballot form.The reality is that following the election, manifesto commitments are discarded because "it is not possible to implement them during this parliament due to mess left by the previous administration". The government of whatever political position, then proceeds to govern according to self interest and dogma and ensuring of course that their expenses, perks, pensions and other benefits of being an MP are safeguarded, or "ring fenced" to use the current jargon. The rest of the people in the country can get on with managing austerity and the effects of business excesses.