Actually, removing a system app won't actually give you more usable storage.
System apps that haven't been updated are stored in the /system partition, which is a read-only partition. System app updates and other apps you install are stored in the /data partition. Each partition has a set amount of space allocated which cannot be changed.
So even if you were to have write access to the /system partition (only possible by rooting), you aren't going to actually use the space gained by removing a system app because no other apps or data will be put there.
That's why when you go to disable a system app, you're prompted with a message saying the app will be restored to its default version. What's happening here is the app updates are first removed from the /data partition, then (I'm presuming) is marked with a Hidden flag.