It's possible that they were talking about or doing speed runs. During a speed run you try to aggro as much as possible, without spamming flash it would be really difficult to maintain aggro on a lot of adds at once. In Brayflox HM speed runs for example, I can tank as many as 10 plus adds on the first pull alone. Without flash spamming it would be almost impossible to rotate aggro attacks on each enemy individually, while the DPS are using aoe attacks on the whole group without loosing aggro to them. Besides using your cool downs during big pulls you are mostly just spamming flash in between riot blade combos to build up your mana for more flash spamming.