As in, you think it's a bad idea to offer it as an option?
Hmmm - how can I explain what I mean. I don't think direct debit for this is appropriate. Of course you could offer it as an option. People are fickle - and come and go, and I doubt many would subscribe to that level of commitment. I just think you'd get better results having a wee intro that showed running costs, and saying that all contributions were welcome. And if they don't contribute, I'll trawl around with my baseball bat!
I'm not being helpful, but just trying to think of a way that might work best.