Of course, your tank doesn't have to be a warrior, and even if it's a paladin or a druid, a DPS warrior can contribute in much the same way. In fact, since neither of those classes uses Devastate for threat, a good DPS warrior can wait until threat is secure and then stack and occasionally refresh Sunder Armor to help increase raid-wide melee DPS. (When Sun goes Arms, he also provides Blood Frenzy to further increase raid-wide melee DPS, of which we have quite a lot, but we don't have him do this if we can't guarantee him a windfury totem.) He or she can keep mobs thunderclapped to lower their attack speed, thus reducing the damage the tank has to eat. And if the tank is a druid who is fighting for threat and can't spare the rage or a paladin who doesn't have the ability, demoralizing shout can be used to lower the boss' attack power for more reduction in tank damage. Even if your tank is a warrior, there are fights where he or she is going to be doing everything possible to keep aggro in a fight where raid DPS must be as high as possible, and having DPS warriors along who know when to use those abilities for the tank so he or she can focus purely on threat generation is a good thing. The amount of damage reduced by the combination of Demo and TC is really quite important for boss fights.
Another reason I love having the DPS warriors around is because I know they're good tanks if called to do it. There are fights where you can find yourself needing four or five tanks in some raids, and knowing that after our two ferals and our paladin tank are spoken for we still have people who can respec and do the job is a relief. Also, no one member of the raid is indispensible by necessity: there have been nights when I've been too sick to raid or otherwise committed (not many, I try to make every raid, but I'm human) and one of our other warriors has respecced, stepped in and done magnificently. In addition, having access to talented DPS warriors has helped my own play when I've specced fury for a night (sometimes you really need to step out of your comfort zone and make sure you're capable of delivering the other role if it ever becomes necessary) which benefits me, anyway. I know it doesn't really matter all that much to the raid.
Our DPS warriors tend to be reliable, solid sources of high DPS. Any fight which allows for high melee DPS contribution, the warriors are always right up there. Last night on Gorefiend, for instance, the kill had two warriors in the top 5, both in excess or 300,000 damage dealt and both close to 400,000. They did this without pulling aggro, despite their lack of a threat dump, because in fact no one is more rigorous about watching raid DPS and threat and warning people than our DPS warriors. They know what it's like to be a tank fighting to stay ahead on threat, and they know that they don't have a dump themselves, so they've stepped ably into the role of 'threat ceiling' for raids where sustained high damage output is important. (An aside, but I should also salute our raid's main feral tank, who also put out some high 300,000+ damage as well. It's not on topic, but she's a tank, and you know how it is, tank solidarity.) If you're putting out more threat than they are, you should immediately fade, vanish, feign, do that thing warlocks do that never seems to work (what is that ability? Soulshatter?) - I kid warlocks because I love them. As long as they're not setting me on fire. That I don't love so much.
In addition to all of this, though... having access to multiple sources of damage mitigation and other shouts, solid DPS, good alternatives if tanks can't make raids or if extra tanks are needed, and an excellent 'don't cross this line' warning system... I like having the DPS warriors in the raid for much the same reason that I like having feral druids and protection paladins. Those two groups provide me with other tanks to discuss tanking with, help refine strategies, and generally support each other in our specific roles as tanks. Having DPS warriors in a run gives me other warriors to talk about specific warrior-aspected issues in any fight. Last night's Gorefiend kill is a perfect example.
To be blunt, I was struggling all night. I couldn't get aggro on trash pulls, I was slow to reposition mobs, it was not my best night as a tank. Knowing we were going to be making our first attempt on a boss I'd never even seen, while I was underperforming to my own personal standards, had me somewhat irritated and snappish. And frankly, thank god for the DPS warriors in that run, because they recognized the problem and talked me down from it. We discussed gearing issues on Gorefiend (eventually coming to the decision that I could gear back from threat and towards stamina in the fight because raid DPS wasn't a concern and people could watch their own threat) and if not for them, I don't know that I would have been able to get through the fight. Not because it was any harder than any other boss (in fact, for a tank Gorefiend is very simple - hold aggro, don't die - since the tank doesn't have to worry about dying, turning into a ghost, and crowd controlling four shadowy monsters that can't be affected by the rest of the raid) but just because they understood exactly what issues were bothering me and could help discuss solutions with me. Sometimes it's just easier to talk to another warrior about DPS pulling threat, even if they are the DPS: they can translate your concerns into "DPSese" and help the raid throttle back just enough to get the kill done, since I had to dump threat gear to make the healers lives bearable.
However, all of this aside, the real reason I like DPS warriors is because Cell looks freaking awesome in DPS gear. Holy crap, I wish I was a hammersmith sometimes so I could use Dragonstrike. Okay, so I also like that when these dropped, they both passed. Thanks, guys.
Next week, since we already have Ready Check for specific boss fights, I'm debating the creation of a general "You're now ready for Hyjal/BT" post covering the gear or stats you should be looking for to walk into those instances, ala our Karazhan guides. Who knows with me, though, I might go off on a tangent about Titan's Grip and fast 2h weapons, I'm unpredictable that way.