What are your thoughts on offering free membership?

What are your thoughts on offering free membership and then having the member pay for services as and when they require them? There is quite a lot of discussion in membership circles about the idea of providing free membership with the member then paying for additional services. Has your organisation tried this? Do you think this model has a place in the future structures of member based organisations? We've already had a fair bit of discussion on our LinkedIn Group. Any other thoughts?