This is a good point. Someone doing mostly-not-effective volunteering isn't there 99% of the time. But that's kind of the point -- they've made a connection and

This is a good point. Someone doing mostly-not-effective volunteering isn't there 99% of the time. But that's kind of the point -- they've made a connection and now care about the project. That's got to have ongoing value in terms of things that'll be more helpful (either valuable help or money). Perhaps even sufficient for people who are there 99% of the time to re-lay some brick and call it even.