It really depends on what your goal is.
There comes a point where certain stats can match and even overtake your primary stat in terms of pure dps increase.
However when it comes to PVP you have to consider that:
A. you won't be slamming into a target & free casting all the time
B. your survival allows you to continue doing damage
C. length of time on a target
With those conditions above it's something you have to work out for yourself & run some testing.
Stacking intellect will give you great amounts of damage but sacrificing stamina, crit, haste and versatility might make you actually weaker when it comes to PVP.
I'd say that for a moonkin you want your starsurges hitting as hard as possible. For this I'd pick a good balance of Int and Versatility. This will help you with damage reduction too.
This will change from bracket to bracket also as there may be items that are worth taking even if they drop intellect... Electrocutioner Lagnut for example gives huge amounts of Versatility which may make up for the loss in intellect.
Finding out your characters stat weights in regards to starsurge damage will be a good thing to do and may help you in deciding pieces of gear.