While choosing NOT to vote may have a firm ethical basis - often times it is merely interpreted as apathy. This is unfortunate. I think in this case, and within our respective systems, a protest vote would be far more productive.
As opposed to saying, 'I don't like X, and I don't like Y, so I'll vote for neither' it's more like saying, 'I'd rather vote for Z'. Here in Canada, we have a multitude of parties ruinning in almost every given riding/district that represent a plethora of issues/platforms: Environmentalists, Socialists, Libertarians, Pro-Cannabis legalisation etc. etc. etc. Often times, enough protest votes are garnered to make these issues valid political concerns.