Just buy it, you'll never convince an uninitiated woman that it's a good idea. Just come home with it one day, if she whines about it just look at her with a condescending sideways look and say "come on, I thought you were smarter than this."
It's a version of "The chewbacca defense", it makes no sense and she won't understand it; but she won't want to challenge the statement either. If she does, shake your head side to side as if you are disappointed in her.
We've all been there, you have to confuse them a bit until they forget about it. After that, if she loves you she'll see how much joy it brings you and hopefully leave you alone.