Processing disclosures takes time and costs money -for the state and organisations.
For volunteer roles I think currently the organisations themselves don't need to pay but it has taken at least one paid employee somewhere to process the applications…also I think for at some organisations it is just for 'peace of mind'.
However disclosure can only show that the person concerned hasn't been caught – and may lead to a false sense of security . Also as far as I am aware there is no expiry date.
It discourages people from volunteering – I know of at least one person who is already disclosed (a teacher) who would be prepared to help out but hasn't got a new application in yet.
In the last year I have had 3 disclosures and I would also question the need for all of these.
Especially for 2 of these (helping out at school trips and covering sickness at sunday school) I would always be in the presence of at least one other adult who has been disclosed. Even for my job (playgroup administrator)I have no direct contact with children (although I do have access to their personal details).