There would be much less of a need for volunteering as a lot of volunteering is to help stem DC’s inability to provide all the required services. For example keeping the streets free from litter should be doable without any need for volunteering.
However, there is still the scenario where a person works in one area but really wants to help in a different area in their spare time. This should be enabled as much as possible and the open nature of OS makes it more possible. There should be status and or monetary rewards for this kind assistance.