Flash is a good filler for when you're out of TP as a MRD/WAR, but you only get like, two or three casts with the much smaller MP pool you have compared to GLA/PLD. You definitely want to learn to be parsimonious with Overpower though, because spamming it hard on every pull will run you dry on TP fast; two per pull is probably fine for most content, but you might need a third if you're in that annoying valley around Cutter's Cry where DPS start picking up momentum and your threat gen hasn't caught up yet.