To step away from the intellectual argument and focus on the practical, one should first note that the answer should be very different depending on the voting system in place.
In a pure first-past-the-post system? Yes, it may well seem that your vote is "wasted". After all there are rarely more than two choices with a realistic chance of winning and frequently only one party (so called "safe seats"). One party can get 0.1% more votes in a given seat then their biggest rival and still take it all.
But in a pure proportional representation system? Well, the party you want in power may only get 1% of the votes... but that translates to 1% of the seats. In circumstances like that it rarely makes sense to not vote.
And even in first past the post systems there's still an incentive to vote.UKIP
are essentially a fringe party in the UK, getting just over 3% of the vote in the last election (an increase on the 2.2% they picked up previously, the 1.5% the time before that, the 0.3% before that and the 0.01% the time before that). They over-perform in Europe (17% the last time) but that can easily be explained both by the way European elections are used as a "pressure valve" to complain about major parties and the fact that Europe is UKIP's main political position.
Yet we are seeing more and more debates about Europe and immigration, two of UKIP's most (in)famous policies, and a hardening of attitudes by the other major parties on the issues... and I'd suggest that's largely down to UKIP.
Because while 3% of the vote overall might no mean much and while UKIP may not have targeted seats in the same way the Green's did when they won their seat, it's still going on a million votes. That's about a million people who have demonstrated they're unhappy with the current major parties and want something else. And major parties want to win elections and want to court such voters. Hence UKIP have power, even if indirectly. It sends a message that UKIP offer the sort of policies that a notable number of people want.Not
voting? You don't send a message. You just don't vote. Find a candidate you do like... there's normally at least one in each constituency... and vote for them. They may not win but when the votes get added up, at least a message has been sent.
Moreover, I think the "all the parties are the same" view is a little too
cynical. Yes, the major parties may be similar and may well be too
similar on certain issues. But on others? In US surely we could all look at examples where a Republican controlled house, senate and presidency would have done things very different to what has happened over the last few years? And in the UK wouldn't Labour have targeted the cuts differently? Reformed schools and the NHS in different ways etc etc?
differences. There may be the number of differences someone wants and what difference there are may not be as wide as others would like but those differences are still there.