I was on the board of a small non-profit. Volunteers with time, enthusiasm and smart ideas are great, but they come and they go. They get bored, they get jobs, they move on. The staff then gets stuck trying to support and maintain the special project that ultimately they can't.

It's hard to say no to someone offering their time and expertise, but unless it is focused as part of a larger, on-going plan, it may do more for the volunteer than the organization.