To me this is a bit of a contradiction. "They do this because it works" and "If you don't vote, you only make the voices of those who DO vote stronger."
I personally see opting to not vote for someone as a vote. You talk about voting to make as much as a difference as possible. Well, choosing to not vote to say "No, your mud slinging tactics don't work for me" is just as powerful as voting for a candidate that you like slightly more than the others.
Choosing not to vote for someone doesn't make anybody's voice stronger. It's 1 less vote against someone just as it's 1 less vote for someone. The voice as a whole dropped by 1 vote.
I vote to not vote for candidates or measures that use dirty tactics in attempts to gain my vote. If you believe that's not going to make any difference, then my vote for whatever candidate won't make any difference either.