Some of them don't pay any rent, or pay at a very low rate to keep the premises occupied if the owner can't find a lessee!
I think they are a great idea for recycling things which might be of use to someone after the original owner has had their use from them, they only become a blight when there are too many of them but they are an improvement on empty or boarded up shops and in some ways they help to keep the high street alive.
The worst blight is out of town shopping centres, (the Tesco effect), where I used to live Tesco's was in the High street with a multi storey car park above it and you could park, browse round the local shops and then come back and do your grocery shopping last, getting a lift up to your floor level, this was a win win situation for the town, there was also a large Sainsbury's just at the back of the high street with a large free car park, both stores benefitted the town instead of drawing customers away from it.
Town planners should have their ears stapled together, or be forced to visit Uxbridge or Slough to see how it should be done to benefit locals and local businesses!
Edit when i left the area there were not many charity shops in Ux or Slough because both towns were thriving!