I can use demons and locks on a fully enchanted weapon as well as long as I activate locks before putting demons on. Since the end result is the same current restriction does not add anything except inconvenience.
The end result isn't the same. Activating locks first means you have to have much higher casting skill to put more spells on top.
One can debate whether or not you should have to have such casting skill, but there is a big difference between activating before or after adding additional spells.