It was not.
https://www.bluetracker.gg/wow/topic/us-en/15973677341--blizzard-gc-quote-and-balance/
We will not be balancing around the existence of dual-spec. The example I used was a case we already have – multiple tank fights, then single tank fights, then multiple tank fights. We didn’t design that around dual-spec. We assumed you’d probably have an OT who tried to do dps on Saph. Dual-spec gives them a way to do better dps.
This is one of the reasons though that we think it’s important to keep pure dps slightly elevated.
Mostly, though, dual-spec is a convenience factor. You can already run back to town and respec in the middle of a raid, and plenty of people did that already (and hated it).
So no. It wasn’t a balancing concern because they explicitly tested without relying upon Dual Spec and they still deliberately made encounters knowing that some Prot Warrior would be swinging away on Sapphiron without much to do. Dual Spec helps, but it was never a point of inflection or concern.
Key takeaway for dual spec: convenience