Demonstrate the efficiency of shadowflare when you fight an add then ? Give me a proof that shadowflare's efficiency in a single-target situation is worth its casting time/animation delay and the loss of ticks resulting from it (as I could have spent that same time casting an attack that does hit).
Do the same for 2 adds ? Is it now worth it ? If not, starting where is it worth it ?

The Hp's of the enemis is a criteria as well, along as the number of players in the party, number of DoTs already in line and some other things.

The "Keep shadowflare up whenever possible" is not an argument I would follow blindly.