My own answer is similar to the one above, but maybe different enough to make it worth writing:
I don't like to be a sucker, or see anyone I care about getting fooled in any way. I'm not against voting any more than I'm against fantasy football, as long as the participants realize and accept that they have no actual influence on policy or the outcomes of sporting events.
In terms of human dignity, democracy is possibly more demeaning than dictatorship. In both cases the ruled has no real control, but in democracy the voter is given an illusion of power. It is like the difference between an armed robber and a con-artist. The end result is the same, but with the con-artist you also feel like an idiot -- insult to injury.
It is important to alert others to the scam. So being a vocal proponent of not voting is more helpful than playing along.