A reason it would be nice to have the ability to create client tags at runtime is for inherited resources. The parent resource might use client tags and the child might not have them. Having the inherited resource run a script to create the necessary tags would make sure they are always there, no matter how many different children there are.
As Phil said way back here, Creating client tags in runtime - #8 by pturmel , client tags are very powerful for [the reasons he gives].
I headed over to http://ideas.inductiveautomation.com/ and the thread I found there on this topic was closed in 2014. I cast my vote and made my case regardless.