Yeah, good question. What if the abortion situation in your country won't change regardless of who is elected? At that point, I would still most likely vote, but my focus would be on non-abortion issues. Remember Greg Koukl's quote: "We should vote in such a way that will get us as close as possible to the political situation we need to be in for the most good to be accomplished."
So practically speaking, if I had to choose between not voting or voting for one of two pro-choice candidates in a close election, I would consider their other positions. If one of them was a bigger protector of freedom of speech and religious liberties, I'd probably vote for him or her.