Honestly its a common mistake I've seen a lot from the writters on wowinsider. They almost all made their columns telling people something along the lines of

"At level 85, it'll likely usurp haste and crit as your secondary stat of choice. But for now, the only way to get it is by reforging another stat into it. The relatively miniscule amount you'll get even from reforging every secondary stat you have simply isn't worth the effort at level 80"

But for some reason they forget that item stat weight is budgeted by blizz. So having mastery already on gear when aquire it is gonna be the very same as reforging it now because Blizz did the reforging for you when they set the stats on the item therefore making their argument pointless.

Mastery is just as good a stat right now as it will be at 85. Sacrificing haste/crit or hit right now or wait for blizz to do it themselves when designing the 85 gear is the same thing except we get to choose which stat we want gone whereas at 85 blizz makes the decision.

Of course, balancing stats comes first, but if you know what you're doing there really is no point in not reforging mastery.