Just think about it this way...your lower erectors can't lift 500 pounds, so you must be working more than just that. Deads bring your hammies, quads, glutes, erectors, traps, forearms, etc...all into the mix. And yes, they're good for cutting, just think of the amount of energy it takes to lift 500 pounds from the floor a few feet, that's massive calories. Plus the amount of muscle it adds is sizeable, which boosts metabolism.
I think people are saying straps can be dangerous because if you can't hold onto a weight, you probably shouldn't be lifting it...as long as you keep your form on point, straps should be fine...but definitely try to go without them if possible. Sometimes the grip slipping can alter your form, so there are risks either way when throwing around such heavy weights.