And there are advantages to voting tactically, in the short term. Largely, if you want to vote someone out** then voting for the next-largest party is the way to do it. Voting for your actual preferred representative is liable to let the person you don't like stay in.
However, in the long term, what you end up left with is a two-party system where you don't _ever_ get change. If you want long term change then you need to make it look feasible that your preferred party can win. And the only way to do that is to vote for them.
If, for instance, you're in a Slightly-Bastard/Really-Bastard area, and you hate both with a fiery passion and want a Lovely candidate, then what you really need is for the Lovely vote to keep increasing each election, so that after five elections of the Lovely vote slowly creeping up, they actually take second place off, at which point tactical voters will switch to voting Lovely to keep the Really-Bastards out.
This may well result in more Really-Bastards winning in the short run. And it's a difficult choice to make, morally speaking. But I certainly don't blame people who refuse to settle for the slightly-less evil with the hope of getting in the much-less-evil in in the long-run***.
*Well, the SNP is one of the enemies in Scotland - but I'm only seeing pressure for tactical voting from the "Anyone but the SNP" slate.
**Under FPTP, of course. With a decent voting system there's much less need for any of this nonsense.
***Doesn't work in the USA, of course, where you seem to be perpetually stuck with a two-party system.
Original post on Dreamwidth - there are comments there.