Annoying as they are as they never give you enought time to actually go through your old stuff, they are a useful way of getting rid of unwanted goods.
I have a rule that if they say 'No bric-a-brac' I have a free bin liner! If they don't want my 'bric-a-brac' then they're not getting anything!
On a more grown up note, a lot of these legit charities are actually suffering from bag theft! Rogue people know when the bags are being circulated and will go around stealing them all before the charities can pick them up. Aparently the charities are losing a lot of money due to this!
What is the world coming to (and not not reply 'Ravel's Bolero
'!