What run speed does
The benefits of a run speed enchant sounds simple. It lets you move out and away from fire faster. But the benefits run even deeper. You get to the adds faster, enabling you to lay down your sweet, sweet DPS on their butts sooner. This reduces pressure on the tank, allowing the tank to increase threat on the big boss. That makes it easier for you to do more damage on the boss. It's the circle of life.
You get out of fire faster. Even getting out of fire one tick of damage more quickly saves the healer some mana. More mana for the healer means that your tank stays alive longer, leading to better raid success.
The point here is to reduce pressure on tanks, healers, and other DPSers. Run speed lets you get to your job faster, reduces risk of death, and helps with the overall performance. As the saying goes, "dead rogues do no damage."
Who doesn't need it
Admittedly, there are some classes that come out of the box with run speed enhancements. Shaman can spec for Ancestral Swiftness, for example. If you already have a run speed talent, then you can probably skip getting an enchant for your boots.
Cooldowns don't really count, though. While they're better than nothing, when you need to get out of fire, taking the time to hit another button, keybind, or mouse key uses up precious time. For that matter, if you happen to need a cooldown that's restricted by a global cooldown, you're totally out of luck.
As a general rule, if you're going to pick up a run speed enhancement, it should be one that's always active.
Where you can get it
Assassin's Step and Lavawalker both include a run speed aspect. You'll notice how two of the highest-end enchants include the run speed; that's another hint to how important it is.
If you can't afford these enchants, there are other options. Earthen Vitality is a cheap enchant that gives you run speed along with 30 stamina. Stamina is useful to everyone. You can usually pick up more than enough materials for this enchant simply by running a few random dungeons.
Considering the huge benefit you get from run speed enchants, there's little reason to not have the run speed. If you've not used run speed so far, try it out. I think you'll see the difference.
Ready Check shares all the strategies and inside information you need to take your raiding to the next level. Be sure to look up our strategy guides to Cataclysm's 5-man instances, and for more healer-centric advice, visit Raid Rx.