Tuning is actually easier if you can use the PTR to get more people to try out the new talents. There is a reason why the first week of a patch usually also has another wave of tuning / bug fixing, simply because of the huge difference of data between something like the alpha / ptr and the life server. So yeah, PTR together with beta would actually make sense.
I remember how much of a shitshow huge talent-tree changing pre-patches like cataclysm or MoP were .... trust me, they want as much data as possible prior to release.

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