Well, first, Flare is one of the most powerfull aoe of the whole game, even with the nerf it received with Heavensward. This reason alone should make you use it as often as possible.
Now for the MP cost, well, use it when you are almost done with your Fire 2 spamming,
Like Fire 2->till almost no MP (just enough for Flare), Flare, you have now 0 MP left, use your skill to switch to Umbral Ice, Ice 3, get back your MP, Fire 3, repeat.

Now you can also use the classic Twin Flare method with
Fire 2 till few MP, Flare, Conversion, Swiftcast, Flare, switch to Ice. Huge damage in short time !