I think one valid point is that the coupling of storage between Email and other services can have rather annoying side effects. In most cases receiving email is

I think one valid point is that the coupling of storage between Email and other services can have rather annoying side effects. In most cases receiving email is much more important than being able to store more Images. Especially as receiving emails is triggered by the outside world, while storing images or files is something you do yourself or have set up in some automated way. Not receiving emails because you messed up something and filled up your storage quota with images seems like a very clear drawback of this kind of combined storage.