I was on the fence about this for awhile. So what about the customers who use Keygen for feature-licensing? They would pay more while the customer who uses Keygen to license their entire product pays way less--not exactly fair. For the time being, I think the flat-monthly-fee works. Though, I'm all for discussing a custom plan if that doesn't work for you, as mentioned on the pricing page. :)

No reason not to offer both models. Pay per use is generally higher than a subscription which explains the benefit of the subscription to the customer. As the provider you sacrifice some overall peak for guaranteed revenue.