[iwwr]

>I think it was someone working on Curiosity that said mars has just enough of an atmosphere that you have to worry about it, but not quite enough to be useful.

It certainly is useful and you can still shed 90-95% of the inbound orbital velocity or even that from an interplanetary trajectory with a reasonable heat shield. It would be far more difficult if Mars were completely airless. You still need retrorockets for landing big payloads but at a discount.

Though the other atmosphered bodies in our solar system: Earth, Venus (cloud deck), Titan can be achieveable with just parachutes.