There is a difference between not voting and spoiling your ballot paper.
If the former, no-one knows why you didn't vote. If the latter then they know you "cared" enough to make the effort but were not happy with the candidates on offer, i.e. you said no to all of them - not turning up does not make the same statement at all, it just says you couldn't give a toss.
Yet politicians have been worrying about dropping turnouts for a while now which is why things like easier postal voting was introduced. Given they have taken action against this problem ( and I understand there are discussions about introducing technology for voting ) but don't seem to worry about taking any action to lower the number of spoilt ballot papers it would seem not voting at all if you are not happy with any of them sends a stronger message than spoiling your paper.