Administratively a nightmare as everyone has a different idea on what is important, or not, as the case may be.
I would love to see (but not actually happen) all those who opt out of defence budget to be rounded up and used as a human shield when the invading army comes a knocking lol oh sorry sir, madam you're not on the protected list, please move along quickly so we can defend the people that matter.
It sounds like a good idea - as long as I can also opt out of all the other stuff the government spends money on that I personally don't agree with I'm all for it. If I can't do that then it's obviously ridiculous.