Individual participants don't want to manipulate their own vote between two candiates, absent external incentives. Incentive compatible is more accurate than non-manipulable. Votes are manipulable through sybil attacks -- voting many times under false identities.

I'm treating majority vote as the status quo to improve upon, with incentive compatibility the basic standard for new mechanisms. Also eliminating vulnerability to sybil attacks would be great, but not high priority.