[vidarh]

Why do think they'd do nothing? Do you think Sinn Fein does "nothing" with the money it receives for it's Westminster MP's?

It's certainly possible for it to be a total waste, but in Sinn Fein's case, for example, whether or not you agree with their goals, it is a very clear principled stand: They don't believe Northern Ireland is rightfully part of the UK, nor that the queen is their rightful head of state, so they can't in good conscience give an oath of loyalty to the queen.

Voters who vote for them know full well that this will the outcome, yet they still vote for them because it accomplishes something to these voters: It (now) gives funds to Sinn Fein and it keeps sending a signal that a substantial number of people in these constituencies see British rule as unjust.

Surely there can be any number of other causes where the signal effect can be preferred by voters who otherwise don't see sufficient difference between the major parties to believe it makes a difference which one of them gets their policy through.

The reality anyway is that in the vast majority of votes in the Commons, the small parties votes have no bearing on the outcome at all because the first past the post system means the big parties has such a disproportionate portion of the seats, so most votes for parties outside the big three are still totally "wasted" by similar logic.