I think that signal (that your vote is available, but no one has attracted it) only "fires," though, if there's evidence the two major parties care -- i.e., that they work hard to win back your vote. Not really sure that's the case (the Democrats could care less that they don't get the vote of many Marxists in blue states, say), but maybe there's some evidence I'm missing.

From a consequentialist perspective, seems like the only relevant thing is whether withholding your vote - or voting for some third party - would actually stand to have an impact. Otherwise it's just a kind of virtue signaling, right?