You're assuming mistakes happen in isolation. I'm assuming they're not. It's that simple.
EDIT: To be clear, the people arguing against you understand two very basic healer concepts. Triage, and Resource Management. We are using Rescue to conserve raise mana and to avoid hardcast raise in the first place. Meaning it's being used in a context where players ARE making enough mistakes to warrant it. Also, it's factually a DPS gain over raise. Not only from the GCDs the DPS lost in the interim AND the weakness penalty, but the GCDs the healer has to use if they're required to hardcast Raise. It's a better tool than Swiftcast if you know what you're doing. And we do. You don't. You wanna be greedy, guess what, sometimes we are too. And it's more satisfying to execute than any DPS rotation in this game, and I play fucking Ninja.