It is indeed what type is for. And well, it'll work for me. So, when you wanted it that way, I'll change it.

However, just for the record: I recommend against it, because maybe in the future, there'll be extensions that might not only extend their own subscription types but also modify existing behavior that is not reflected by subscription_type. And as you know, we cannot change an interface so easily as compared to code within CKAN. The passing of subscription_definition allows greater flexibility than passing a dedicated variable whose meaning and content is not likely to change.