Pvp casters interrupted by direct damage

[Twizffxi]

Sounds like a game over for the caster what's the thought behind this?

Without Interupt on Physical
Caster starts casting Cure
Melee Hits Caster for 25dmg
Caster is Healed for 100
Caster cast DoT spell on Melee
Caster Uses Sprint
Caster Laughs and runs away
Melee Facepalms
Repeat Add Nausium

With Interupt on Physical
Caster starts casting Cure
Melee Hits Caster for 25dmg
Caster is Interupted
Caster Uses Swiftcast
Caster cast Cure
Caster Is Healed for 100
Caster uses Sprint
Caster Laughs and runs away
Melee Chases after to prevent more casting but can't use Sprint to catch up.

Without there is no need for the caster to fear getting Hit by Melee. Simple.