Shadowflare + Swiftcast

[Lyrica_Ashtine]

Wouldn't it just be best to judge the situation on whether to save swift cast or not?

There's no way to "judge" when you should save Swift Cast or not. The need to use Swift Cast for resurrect doesn't occur till anyone actually messes up and dies. This will lead to two situations:
1) You used Swift Cast earlier to maximize your DPS
2) Swift Cast has been off CD for a long, long time

Much like your buffing skills (Raging Strike), you try to keep it on CD to maximize your DPS.