A few points need to be made here:
(1) Parties aren't the problem. The strictures of governance to which all parties in Parliment must abide are the problem, for the most part because they cannot be reformed by parties themselves in the absence of social pressures on markets.
The democratic gains we now enjoy were made in the early twentieth century when perceived threats to markets (eg, depressions, labour activists, communists, etc), forced business to make consessions. Sometimes more progressive parties were voted in as well, but not always. The parties weren't responsible for the change. The gains culminated in the 1960s (eg, healthcare) but by the time things calmed down in the 1970s these gains were already being clawed back. Again, it hasn't mattered which party has been in power. Sometimes things have been worse under the more "progressive" parties. Parties aren't the problem.
Today there is no real source of social pressure on markets, so voting for any party is a vote for the further retraction of democratic gains. A fixation on ousting Harper and the Conservatives is the surest way to entrench the continuation of conservative governance.
(2) Abstention does not communicate a motive to government, so there is no way to tell if people abstain out of habit or principle. Problem is, the same is true of voting. There is nothing about marking a ballot that communicates that a voter has risen above habit - the habit of voting - or disinformation.
Voting has two main functions: determining which party gets seats in Parliament, and indicating the level of legitimacy upon which Parliment stands.
The political classes are very sensitive to voter turnout for this reason. But if, as they often protest, abstention is meaningless because governments (progressive or conservative) respond to low voter turnout with business as usual then it shows that Parliament was never interested in nor deserving of our votes in the first place.
(3) Voting is cynical and apathetic. The most cynical thing we as citizens can do is resign ourselves to voting for the lesser evil because we see top-down policy decisions as our only realistic and effective recourse to democracy. Likewise, what could be more apathetic than hoping impotently that despite unchanging conditions this next round of electoral endorsements is somehow going to present a different outcome. We honour those who struggle for democracy by rejecting the cynicism and apathy of voting.
In the context of the ongoing erosion of democracy, and in the absence of social pressures on markets, revoking one's edorsement of legitimacy - abstention - is a responsible choice.