I think potions have the 3rd lvl req because of their convenience. I think its strictly a game balance reason.
As far as in game pros to potions is that anyone can use it regardless of class level, while a scroll has the requirements you listed above.
Another thing you might have overlooked is that you need to cast the spell from the scroll, so if the spell is a full round type of action or multiple rounds, you still need to spend that time casting it, while a potion is just one action.
