Bitcoin is changing things. You can embrace the change or you can fight against it and cling to old models. In the long run the former is going to be more profitable.
As much as I agree with this statement, I think monthly subscription payments are familiar to the majority of 'normal' customers, and having familiarity is really important for acceptance (of your site/service, not of bitcoin). I wish it wasn't like this but look at how apple has 'such great interface' when really the main thing they've done is make new situations feel familiar. I'm not saying being able to offer monthly subscriptions will be the deal breaker for your service, but every bit of familiarity you can provide makes your customers feel that much more at home using your service. True, prepaid is also familiar, but I think prepaid cannot be directly substituted for monthly payments. It is, however, probably the closest alternative.
This is not to say I think bitcoin should 'natively' support subscription payments. I am currently trying to figure out this same subscription-with-bitcoins thing myself, and have no answer. I will be making my customers make a payment once a month.
I considered the prepaid model as an alternative but it has some drawbacks, especially from the business side of things as already stated.