I once had a phone with a removable back, and a special case designed to hold a massive battery in place without the back plate. Sucker was almost like a brick in your hand, but I've got huge hands, and I LOVED the fact that my battery would last for nearly a week with intense usage(I read books, like, all the freaking time on my phone)

Actually, no. Due to the thinness, I can add a protector that adds just enough so the thickness feels right to me. A thicker phone would necessitate a thicker protector that would make it difficult remove the phone from my front left pocket where I always carry it.

One might make the argument that a thicker phone could be more durable and not require such a protector.

A thicker phone (presumably due to a larger battery) doesn't necessarily solve the problem of fragile front and back materials (glass).

Since I'm the most clumsy person of all time, I have to add these two things to any phone. It being pretty thin, it still fits my hand pretty good.

Not if you're factoring that in with the thinness. I've bought a thinner phone over a larger phone specifically because I know that the total size of the phone + case fits in my pocket better. Especially when using something like a Lifeproof/Otterbox/Tank case...

It doesn’t defeat the purpose of the thinness, most obviously because screen protectors and cases are optional.