I maintain that it is massively irresponsible to refuse to vote blue because you can't have your preferred candidate. The game sucks, but we have to play it. I don't like it, either.1
As Lawrence O'Donnell noted, the Democratic Party ignores the Left because the Left always votes for them.
It is not an inherently unreasonable position to say, "If you don't support what I have clearly said I want"--whether this is candidate- or policy-based is not relevant--"why should I support what you want?"
And having had it happen twice in my lifetime (though only once when I was old enough to remember) that vote-splitting between two non-conservative parties handed unbridled power to conservatives, and then seeing this used by the less-principled non-conservative party to snag votes away from the more-principled one (because the former has a much larger hard base than the latter), and
now seeing that same party quash a clear recommendation by a Parliamentary committee to have a referendum between our current electoral system and a proportional representation system, because that would obviate all their fearmongering that they use to get votes from swing voters... suffice it to say that I really, really sympathize.
(If you look at Canadian federal elections in 2006, 2008 and 2011, the share of the popular vote among parties getting either 5% or electing one member enjoyed by left-wing parties--Liberals, New Democrats, and Greens in the latter two--went from 50.5% in 2006, to 51.8% in 2008, to 53.9% in 2011. But each time the Conservatives got more seats, winning a majority in 2011. Then a bunch of NDP and Green voters went for the Liberals in 2015 because of Justin Trudeau's promise to bring in proportional representation and the realization, once again, that the NDP and Greens can't crack the Liberals' ~18% base... on which he promptly reneged when Parliament didn't recommend the system he wanted, and used the majority those same voters, who voted for him on the promise of proportional representation, had given him to kill any chance at getting proportional representation. Goddammit I despise the Liberal Party, very nearly as much as I despise the Conservative Party.)