Presumably once the game goes live on Friday, you'll be paying $60 for the game WITHOUT the added items. Hence, the added items from the Anarchy Edition are a "free" upgrade for those who pre-order.
Unless you're not aware that "pre-order" and "pre-load" are one and the same thing on Steam - seeing as it would be a bit silly to purchase the game and then wait until friday to download the game. Considering that Steam allows you to load it as soon as you buy it, and all.
But yes, the only real benefit of getting RAGE via Steam seems to be the instant playability the minute the game launches; personally I don't see that being worth R150 more, but to some people it's a must.