In dungeons Flash is still great for grabbing mobs out of Overpower range, when you're pacified, need to conserve TP, etc, esp if you don't have 5 stacks (and Steel Cyclone is far from being an on-demand AoE ability. Using Infuriate on Unchained is so much better for groups when its up). Flash is also really good when making large pulls on the move, you don't have to worry about being in the perfect position and can start the real fight at full TP.
In Raid, it has less utility (except certain ones, like T4 and A2S), but what would you use in place? Featherfoot is useless, Second Wind isn't nearly as good as Equilibrium (I mean, you don't want Flash for the same reason, why would you want this?), Mantra is a meh cross-class for everyone, Savage Blade and Haymaker...just lol.
