On the contrary having several manifestos used by a coalition actually means that there are many more people who get some of the policies they voted for passed. Under FPTP only a minority has ever voted for the manifesto that wins and gets implemented. If there is a coalition created by PR then more than 50% of the electorate will be getting a large amount of the policies they voted for implemented.  The whole issue of manifesto promises also makes the assumption that parties always stick to them when they get into power. This is not the case even under single party government. Election promises are often not implemented as politicians are simply using them to win an election, they may realise that the policy will not form the basis of a sensible government policy, or be too politically difficult to implement. Creation of a democratically elected House of Lords was in every New Labour manifesto, yet after three terms in power was at best half complete.(Summers, Labour’s attempts to reform the House of Lords)