well, for me...
it's more about the method than the goal.

I've been asked many times, "can you spare a minute?". Yes, I can spare many. But it's money you really want me to commit; of which I really can't spare much because I'm currently unemployed. I think that's what gives me a nagging feeling of being misled*. I don't like explaining to complete strangers that I'm unemployed, and frankly I don't much like reminding myself either. While technically I could afford a small regular donation to your noble causes, that would only become affordable by cancelling one of my existing contributions**.

Also; some of the causes, while noble, might trigger misplaced resentment. We're not taxed lightly, and 28% of the UK's annual spending*** (£190bn) goes on social protection, and yet we're still missing some needs. In an ideal world, we wouldn't be wasting £30bn (that's 4% of all expenditure) on servicing national debt (and that's just interest, not paying it back), we wouldn't need to drop £38bn (6%) on defence; and in any world, we certainly shouldn't need charity to ensure that children aren't homeless.

* maybe it's my not-very-crippling long-diagnosed Aspergers.
** all charities that originally asked for cash using less socially-pressured means.
*** based on projections for 2009-10.
(The Incredible Sulk Finally remembered his password on, Thu 27 May 2010, 23:44, Reply)