I honestly don't get the point of deliberately spoiling your vote. They don't differentiate between someone who writes them an essay and attaches it to the ballot and someone who accidentally ticks both boxes or scribbles somewhere on the ballot paper. They'll all simply counted as 'spoiled' and not counted.
I think most people that spoil their vote do it as a means to protest. However, as you said, due to the way the counters don't differentiate spoiled votes in anyway, the vote is effectively wasted and you may as well have stayed at home. I guess the hope is that if enough people spoil their vote such that the count results in an unprecedented number of spoiled votes it may be inferred that the voters were dissatisfied with the vote.
I don't want to derail this thread so coming back to this particular Referendum I feel that we have been put in an awkward situation. The Yes vote could have been for something much more favourable if the Fiscal Treaty is renegotiated/restructured towards growth as Hollande seems to favour. Voting No makes us look bad, again, and could put us in a tricky situation as Beebub outlined above.
It seems they will not hold a second referendum
. To me, it looks like they've set it up for a mess, either way. I have never spoiled a vote and in this case I don't think it would make any impact. I'm regrettably leaning towards a Yes vote.