Here's how the average person thinks of it...from J.S. Mill..."It is, of course, not desirable that anything should be done by funds derived from compulsory taxation, which is already sufficiently well done by individual liberality."
What is sufficiently well done by individual liberty? Let's find out. If we allow taxpayers to directly allocate their taxes...then it's doubtful that they will pay the government to do anything that the private sector does sufficiently well. This is because everybody wants the most goods for the least amount of life. Which is exactly why we really need to apply this concept to the public sector.