So how would one indicate a lack of confidence in the system, as opposed to the specific candidates? Abstaining is not simply a way of expressing apathy; it can also indicate that one finds the office itself illegitimate.
...That is an excellent point.
Also, from a pragmatic point-of-view, spoiled ballots tend to be reported and counted exactly the same as uncast ballots, so showing up just to cast a spoiled ballot is a complete waste of time (yours and the officials') regardless of the reason.
Well, Toronto just finished its mayoral election, which was decided by a margin of almost 100K people (out of the roughly 500K votes that seemed to be counted for the mayor's office). My showing up (to vote not for the guy I thought would do the best job, but in typically Canadian fashion, against the guy I thought would do the worst) feels like it was a waste of my time, other than councillor, where i did vote for the winner. School board trustee, they gave me the wrong ballot, so I had to abstain there.
Basically, I don’t think showing up and spoiling your ballot is a waste of time; it’s speaking up and registering your discontent. I know they’re counted the same as uncast ballots, but maybe they shouldn’t be.