I use disengage in every ICC fight except the gunship. It's a very fast way to move and not delay shots.
I don't try to use it for running straight faster though. I'm a combo mouse and keyboard mover. It works well for me. And running without stuttering your movement delays auto shots and lowers your DPS. So I'll usually do a slightly not straight move. I run straight, stutter, strafe-turn-disengage then mouse-turn back. Depending on cooldowns, I might also let steady-shot turn me as its castbar progresses.
Here's how I use disengage in ICC.
There is often a lot of distance to cover. Plus it's great to clear frost lines.
I generally DPS LDW and am responsible for the back guy (who is tanked). I hit disengage after DPS'ng and mouse turn in midair with a MD on the rear tank. After downing the rear mob, I quite literally strafe some side mobs on the return to the boss.
Disengage straight back if my bloodbeast gets too close. This happens surprisingly little. But in a pinch it's great with a slow trap and a concussion shot.
Move out of center in one click, just aim your butt for something not overly green.
If I'm not the collapse spot, it's how I move to my spot after a collapse. Quick rotate, disengage, quick rotate (possibly via steady shot which will rotate you while winding your cast bar).
No specific use here, but it's often to get in range of a dps target instead of moving away. You do have to choose one or another and it's probably just easier for me to properly disengage when I'm under less pressure. I do occasionally use it in the final burn phase.
I'm usually on kiting and safe-killing duty of the explosive zombies. It's great for kiting. It's also great for getting to DPS a blazing skeleton on the other side of the very large room.
I'm not usually on kinetic bombs. One of the luxuries of having a bunch of hunters. So I use it to get to far range of the everyone-spread-out shock vortex.
Again, I use it to get to far range for a spread-out before the fear timer hits. Basically it just gives me a little more time DPS'ng the boss.
I haven't killed her in 25man yet. Chalk it up to business travel and light attendance? But it's pretty common to pop it after the pull-in.
Supposedly if you are carried by the Valkyries, you can get a priests' levitate and disengage back from the cliff if they drop you. I tried to do it without a friendly priest and it just launched me under the platform for a long long time. I did use it in other ways. But I used it sparingly. I used it for positioning on killing the Valkyries. I used it in killing and kiting orbs. I used it when a Valkyrie grabbed someone and then we had to move for the black ooze. But that's luck of the timer. Generally, I didn't use it anywhere near a ledge. With a 25 second cooldown on Disengage and a 15 minute plus fight you could use it a silly number of times.