I feel kinda like I must be missing something basic and fundamental as to why we'd ever want to Swiftcast Flare over F4 on a single target.
In the second rotation in the video, assuming you get a Firestarter proc to cover your SC, both would be cast in the same 1 GCD window and F4 would have 20 more base potency than Flare in that ~2.4 second timeframe.

While flare gains more from the Swiftcast(-1.5 seconds give or take) than F4(-.5 seconds) in the end that SCFlare GCD would still be doing less potency than a SCF4 given they're both in the same GCD.
...I think. Is there some obvious modifier I'm forgetting or not seeing?