As I said before there are a large % of the population that will never vote, irrespective of anything. I've set this figure at 15%, as that's how many people didn't turnout for the Scottish referendum, a vote that was significant for the Country and each vote did (potentially) have a direct impact on an important outcome. This 15% would represent around 7 million people that all parties will never be fussed about. Short of offering these 7 million people a wod of cash for their vote, they won't bother.
So the question is how to get the 20% that probably would vote if they felt their vote would make any difference or if they felt there was a party that was worth voting for. That's around 9 million people that don't bother voting because they're disenfranchised.
Now there's a big difference between politicians thinking there are 16 million people who will never vote regardless and therefore there's no reason to waste any time on them; versus them thinking that 9 million of them would vote for their parties if their views were heard. Spoiling the ballot changes your stance from "ungettable" to "potentially gettable" and they'll canvas why you aren't voting and what they'd have to do to get your vote.
Just like deciding to vote Green or UKIP in 99% of constituencies has zero effect on anything, but if millions of people do it i can have an effect on policy. With the moronic FPTP system the vast majority of votes mean nothing anyway; so either you think voting is worthwhile and therefore spoiling is worthwhile, or you think voting is pointless and therefore spoiling is pointless. The bolded statement you could say about any small party: why bother forming at all as unless you get a 30x increase in popularity you won't be a priority.
TL;DR If you believe that voting for the vast majority of the population is a worthwhile endeavour; so is spoiling.
Firstly, the claim 9 million voters are disenfranchised is a bit spurious. In my experience party supporters who don't bother to turn out is a far bigger problem than swing voters who don't connect with any of the parties. Plenty of people keep an open mind til voting time for sure, but in terms of being so wholly disenfranchised that they want to vote but find that none of the 5 parties represent their views? Comparatively rare from my experience in an admittedly very safe seat.
Secondly, think also about the first point I made.
No-one know who the spoiled votes represent. That's crucial, and its the reason that spoiling your vote is different from voting for a minority party. In essence you're hiding with a group of people who may all have different points of view. Non-voters are easy to find, they didn't get marked off in the first place. People who vote for a party have their views represented in the totality of the vote for each party. Spoilers have neither of these. Hell, spoilers don't even know if other spoilers agree with them.
Which leaves two options for the main parties or the electoral commission to try and figure out what you mean with your spoiled vote. There's random sampling, but polling an unknown quantity is exceedingly hard when we're talking about an appreciable proportion of the country's population who voted. To all intents and purposes you have to canvass a significant proportion of all voters just to pin down some of the people who spoiled. Even then you have one hell of a piece of work mapping out demographics and reasons for spoiler voting, since we're talking about 10s or maybe 100s of 1000s of people to make the study significant.
The other option is to assume that the spoilers are just typically representative of the area in which they spoiled. ie white middle class people spoiled their votes in white middle class areas, and so on. That brings an obvious problem. If you find that, say, 10% of voters in Chipping Norton are spoiling their ballot, you would assume that those 10% are largely part of the Chipping Norton set, who may have beef with the Tories at the mo, but would jump off a cliff sooner than vote Labour. Likewise the Tories will assume that spoilers in Scotland are not potential Tory voters. A spoiled vote says nothing. However if the Greens added 10% to their vote in Chipping Norton, that would be one hell of a message.
On the other hand if potential spoilers all vote for a party that's close to their political tendencies then we don't have all of that jazz, its much clearer where you're going. I see little value in playing hard to get, when its
so hard to get. If, say, UKIP hit 15% of the national vote in this election, then the Tories will sure as hell be adjusting their policies to take account of them.
Spoiling your vote leaves you hoping that the other spoilers feel the same as you, and hoping that the main parties take the effort to try and figure out who you are & why you spoiled. Voting for a minority party makes it clear as day how you feel & where your views lie, and makes it easy you to figure our whether your view is matched by others.