My workplace has paid staff and also volunteers. Many staff members also volunteer for other third sector orgs. If you think that the service can be provided just as adequately with no paid staff, and just the support of volunteers, then I would urge you to give it a go yourself.
Volunteers are a very very valuable resource, but for many reasons, they just aren't as reliable as staff, they lack the level of dedication that comes with relying on an organisation to pay your bills rather than just doing it our of the goodness of your own heart.
With cuts left right and centre to local authority services and support that the government deems not important, the third sector is picking up a lot of slack and doing a shit load more work for a lot less money. So yes, I do think that people deserve decent pay for doing a potentially stressful, difficult, responsible job.