CD would be a terrible idea. Like it’s been pointed out, It would ruin the whole point of it. Dual spec isn’t just for PvPers, it’s there so that it’s easier for people to join other groups in a multitude of content.
Hey I want to do dungeons with my friends guess I’ll go tank for them, then I’ll hit BGs with guildies later, then back PvE spec for raid. With a 6hr CD, I would probably just sit raid logging (same as if dual spec was never there).
By removing barriers from people enjoying the content they want you boost participation and there suddenly is just a whole lot more to do in general.