It's not just a waste of time. Voting in a rigged system legitimizes the system because if a lot of people are seen voting it creates the perception that it is legitimate (from the fact that the people voting seem to believe it is).

Also, this push for as many people voting as possible is BS. They say it's to maintain or improve democracy but it actually does the opposite. If someone doesn't know anything about the issues or the people running, they absolutely should not be voting. What ends up happening is they just vote for the candidate with the most ads (because they may have seen them on TV the most) or the one with a name they recognize (usually the incumbent). The whole point of democracy is that if you have politicians that screwed up, you can vote them out but if you have a bunch of people voting that don't know what's going on, they get in the way of that.

However, the real reason they push as many people to vote as possible is because it makes it easier for them to cheat. In that, if you have a polling station where there is almost no one, but you have a drop of thousands of votes, completely inconsistent with the number of people that showed up to that polling station, it becomes pretty clear there was fraud. But if you have a crowd that is difficult to count, no one really knows how many people voted so it makes it easier for them to slip in fraudulent votes.