It would be very convenient to be able to have the option to apply a tag or tags to a Quality Profile. As it is, as far as I understand it, once I create a tag I have to go through and apply that tag to, in my case, the (many) series that I want to apply it to (but not all). But all the series I want to apply the tag are already assigned the same QP and adding the tag there could affect the change immediately, done and done. For some people this may involve cloning some QPs to apply different tags to in order to achieve their particular requirements, but that’s still so much easier than applying and manipulating tags for series individually, and seems like a sound logical and organizational approach.
The Series Editor allows bulk changes to series that could make the task somewhat easier, but still a manual approach, however for now at least, while QPs can be changed there, tags cannot. I think there are requests for this out there already. Not the ideal approach in my opinion.
I suppose allowing tags in QPs would mean tags could exist in QPs and in the series directly, but it seems a straightforward rule for this possibility could be made to apply, even provide more flexibility. For instance tags applied to a series directly could add to those in the QP that’s assigned to it. If the tag already exists there, its just a NOP.