The reason I can say that is because tanking is just another form of crowd control. The heart of CC is in keeping mobs controlled so that they don't melt faces or eat people while you wear them down, and that's exactly what any tank, be he warrior, paladin or druid in bear form does. All tanking is just a subsection of CC, it's no more an independent role in an instance than saying that we have CC, Magic Damage, Melee Damage and Healing, or going even further and differentiating the type of healing we have. A lot of times, a group will take a second tank into Shattered Halls for the express purpose of using that second tank to help tie up some adds: she's just performing a CC role. When you think of the game in this way, divided into this new 'Holy Trinity' you suddenly get whole new ideas about what's a hybrid and what is not.
Mages? CC and DPS. Rogues? CC and DPS. Warriors? CC and DPS. Each of these classes does their job differently, of course. Mages CC by totally immobilizing their targets with a renewable effect that has diminishing returns. Rogues apply a non-renewable form of CC that generally lasts longer than the mages. And warriors CC by keeping the mob or mobs focused on them, a much longer term form of control but one that requires healing and damage to their controlled mob or mobs. Tanking is at once the most versatile and most limited form of crowd control, because while it can be applied to multiple mobs at once and doesn't automatically break when someone else hits the tanked mobs (indeed, the whole point is to hit them) it also requires the most work from the other classes as it calls for them to focus on the tanked mobs and burn them down with damage while keeping the tank up.
What's the point, you might well be asking? Well, the point is that as you're running the endgame instances, you have multiple classes doing the tanking job. Therefore, it's time to consider the warrior as a CC class. You may already have a feral tank, enhancement shammy, paladin healer and hunter ready to go for your Shattered Halls run, and a fury warrior may NOT in fact be the worst choice for a fifth to fill out the run. The hunter will be trapping and DPSing, the shammy will be DPSing, the bear tanking, the pally healing... and when the fury warrior is not DPSing a boss, he can CC by offtanking. Sure, it means his DPS will be lower on the trash. But you HAVE enough DPS for the trash. What you need is crowd control, and simply by equipping some of his tanking gear and soaking up the occasional heal, that warrior can indefinitely control that mob. You don't have to worry about a sheep breaking, or a sap wearing off, or re-trapping, and while tanking the mob the warrior will also be damaging it, reducing its armor, and otherwise softening it up so that when the party is ready to move onto it, it will go down rapidly.
Yeah, warriors as CC is a weird idea. But it's a weird idea that needs to be considered in the era of multiple tanks. If you want to have well-geared warrior tanks for endgame, you need to bring warriors to five mans who may not be geared well enough to MT them, or unable to respec to a build that's really only useful inside instances. An Arms/Fury warrior may not be set up to establish as much aggro as fast as a full prot spec warrior can, but given ten or more uninterrupted seconds with a mob he can stick it to himself like glue. It's time to fully consider the offtank as a CC option for the good of warrior tanking as a whole.