Two separate chats today which made me come to this conclusion: Volunteering is about Giving. There should be no issue when people choose to volunteer on a long-term basis or ad-hoc basis. We have to deal with it, and accept what they can give.
The National Volunteer & Philantrophy Centre (NVPC) has this publication on Engaging Ad-Hoc Volunteers: A Guide for Non-profit Organisations. The way I see it, for the organisation, ad-hoc volunteers can provide that needed skill or expertise, and manpower when you really need it. On the other hand, for the ad-hoc volunteers, it’s a way for them to contribute to the community based on their own time and target and meet like-minded people.
Personally, I think it is really great if volunteers can stay on beyond that particular project. I find it more meaningful if ad-hoc volunteers can transform to become more regular volunteers. And for me, I just find it easier to interact with people who volunteer regularly, or ad-hoc at smaller events, rather than ad-hoc at larger events/ projects. Somehow, I feel they are more committed and genuine to help, contribute their time and energy, without really bothering themselves with all the marketing and branding of the larger scale events/ projects. I like the simplicity of mind in that kind of ad-hoc volunteers.