Flash itself is not the problem. The problem is the lack of other AoE threat generating tools which PLD has access to. Nothing about Flash or the concept of it implies that it should do damage.
They really need to put War Drum back in the game. Paladin's shield is mostly aesthetic at this point. Not that we don't block attacks with it, but rather that there's really not much interaction with our shield outside of shield lobbing/swiping which are pretty situational. Why not give back War Drum which will almost always be useful, even in a ST situation War Drum would be a better choice after a block than Shield Swipe almost every time as additional threat is usually more useful than a pacify that the boss is either immune or completely DR'd on.
I understand a lot of people's complaints about PLD is not just AoE threat but AoE damage compared to the other tanks, but I feel like that is a much less valid complaint when discussing tanking and tank based abilities where the focus should be on tank focused stuff not damage (and I realize it is SE that has turned every role into a quasi-DPS, but I still feel that tanking stuff is more important to tanks than damage stuff).