Simply put, for smn time is damage, the more time you save the more damage you do. Throwing quickcast up, then shadow flare lets you get to your rotation all that much faster.

It makes you look like you're doing, it's neater, and it gives you more time to get everything else up.

And the final nail in the coffin, there's no reason NOT to use it for Shadowflare. Unless your Egi died and you need to get it back fast, or the party is in deep shit and you need to raise a whm, there's no other real pressing use for swiftcast.

So meh, it's a moot point.