Ugh, they drive me up the wall as well. I can see a place for them, in increasing the donations for a charity, but they're always very pushy and tend to canvas a lot where they know people are more vulnerable. Plus they take quite a large percentage of the donations.
I have actually started with a positive response a couple times before - both were charities I had an interest in, and I was happy enough to donate something. (Though both times they insisted on giving me the entire ten minute spiel on all the good the charity does, despite me trying to cut to the chase!) The first was a few years ago - I signed up to donate £10 per month, then had a phone call after about 3 months to thank me and explain again about all the good they do, and how they could do more if I could up my donation by £20 per month. No - £15 then? £10? £5? But what about the people and animals who so desperately need help? Didn't I want to help just that little bit more?
I cut them off in the end, and cancelled my direct debit. (Though I did feel a little guilty as not the charity's fault).
The second was a couple of weeks ago - same again, would be interested in donating, but had to listen to the spiel. This time I was uncomfortable from the start as I live in an area with many elderly people, several of whom are ill or vulnerable, and the speech was very clearly aimed at playing on the fears of those people. (This charity provide community ambulances and defibrillators, and what with the NHS on its knees.....). I also had to give all personal and bank details there and then apparently, and it would not be possible to do it online or at a later date. I did later check company name and it seems legit, however the end result was that the chugger was too pushy and (I felt) manipulative, so if I donate to the charity I'll be looking into and way of doing it direct.