Couple points:
1) I am still leveling and gearing up, so sometimes I am very far behind the DPS in the gear power curve. I did the Vault for the first time yesterday, and I was the only one who didn't get sync'd down. I didn't spam it, but I added a Flash to every initial Overpower on trash packs. Nobody seemed to mind, or if they did, they didn't mind enough to say anything. Maybe they weren't expecting me to do much besides hold AE aggro.
2) I do understand my rotations, but not every DPS understands which order to take mobs down, even with marks. Sometimes, in that 4-6 mob pull, DPS fixates on mob 5 while you are working rotations and moving down the line. When a mob turns to run off and I don't want to interrupt a rotation, Flash works just fine. It's on my bar for a reason, same as Provoke, which might get used once every 3-4 DF runs.
3) I screw up plenty, but not everything is based on my screw up. If I mark 1-2-3, and DPS starts with 3, that's not on me, but rather than adhere to some hard and fast rotation rule for the sake of forum purity, I may hit Flash. Once again, I use it infrequently enough that I almost always have full MP when I do use it, and it doesn't interrupt any of my TP combos. It can be and is very handy that way.
4) Sometimes, the DPS is such that it's all big pulls and sick AE. My job at that point changes from proper enmity rotations to flinging Tomahawks, running between packs with Flash (since it is PBAE, insta-cast and doesn't require a target to work properly), and then simply holding AE enmity while my DPS does their AE "nuke the world" thing. And even then, I can't spam Flash because 4-5 casts puts a WAR OOMP. I just mix it in to make my DF life easier sometimes.
Bottom line, there are times where Flash is the perfect tool for the job. It is not keybound to any of my G13 keys and I have to actually click it, so it is a background tool, but a tool that has uses nonetheless.