There are a few things that a case can be made for not impinging on bag space. These might include:
Treasure maps
Unlearned recipes
Quest items like disguises
Soul gems
But I expect that the developers considered at great length this topic and have reasons that seem good to them for setting it up the way it is now
Just FYI, the regular treasure maps are total junk. Youll only get a green from it. The collecyors edition maps are better, but still mostly junk to deconstruct for materials. The blues that drop have NO TRAITS! Junk....