I think it was really nice of Blizzard to cook up a way to fix our bag space problem with the toy box and primary stat conversion. Recognizing this, I propose a way to make it even worse.
I'll start with a story. There was a brief period on the Mists of Pandaria beta where the combination of the feral 4-piece PvP bonus, the Feline Swiftness talent, and a run speed enchant resulted in the glory that was the 132% speed bear. I could run around the first boss' room in Stormstout kiting a howling pack of monkeys before the DPS even made it up the stairs, and zooming around Pandaria's countryside was equally fun. Run speed's value was obvious and it was a bummer when the PvP bonus no longer stacked with Feline Swiftness.
So now that run speed, cleave, and "multistrike" are on the table as randomly-generated tertiary stats to be found (if rarely) on gear, I'll be blunt: I want enough cleave while healing that my Rejuvenation will hit, if possible, the people in local hospitals. I want enough cleave while tanking that Thrash will run out of targets in the game, physically reach through the computer and smack the people who don't read raid chat instructions in LFR. I want enough run speed that I can cap the Warsong Gulch flag before the match even starts, triggering a singularity that will cause time to fold in on itself.
Here's the rub. Knowing that tertiary stats are going to be completely random and also rare, the only way I can stack any of them to a useful level is to hoard whatever pieces I can find and create specialized sets for them. It'll be great to free up all the bag space currently devoted to our offspec gear, but how long will those bags stay empty with the siren call of a "speed set" or a "cleave set?"