Type 7 makes it possible to bring in external routes in your routing system (i.e. backbone area) THROUGH a stub area WITHOUT breaking the stub area (i.e.: no Type 5). After passing the ABR of the NSSA, the Type 7 will be advertised as a Type 5 in the rest of the system.
They could just as easily have used type 5 in NSSA and made the distinction be that ABRs will not repeat Type5 from other areas into an NSSA, but will repeat Type5 from NSSA into other areas.
I'm not making an argument - my post is simply musings on why it was chosen to be done by defining a new construct which is for all intents and purposes the same thing.
Also of note is that in addition to blocking Type5 LSAs, stub/NSSA areas also do not get the Type4 LSAs which point to the ASBRs.