Flash spam is important in some end game situations, but basically it is a mix. Like someone already said, it won't always keep threat alone, and it's also resource intensive. Scorn and Spirits Within should generally be used on cooldown because they're free, and you have to tab through targets constantly to find anything you're about to lose threat on. If you have one target that you're about to lose threat on and the other targets are OK, you should do a rage of halone combo on that target. If you're close to losing threat on two targets, you can split the savage blade and rage of halone between them and cycle the combo again (though this isn't always the best way to do it, it really depends on how fast you're losing threat). If there's three or more targets you're losing threat on, you probably need to flash spam with some riot blades mixed in if you're low on MP / without ballad.

So yes, Flash spam definitely has a place, but it should be avoided in any situation where rage of halone combos are sufficient, because they don't cost MP. Really though you need to be very dynamic as a tank. DPS and healing change from fight to fight, even in a static, so there is no go-to strategy or perfect rotation that will keep threat in every situation. These abilities all have a time and place, and as with any class it takes a little personal experience to understand when to use each, and also when you don't need to use them. Don't be afraid to keep threat close, either - at end game you'll need to conserve your resources as much as possible. So if you're only a few hundred threat above a DPS or healer, that's good enough, so long as you're confident you can keep it that way. That just takes practice.