It's obvious rubbish, but hey - that's humanity for you. Take Duels Of Planeswalkers game for example - you can pay 0.99 to unlock each deck (there's like 13 of them), or you can just play the bloody game, enjoy it thoroughly and in few days time you'll unlock decks for free. Nonsense to pay, right? But there are people who buy it, allegedly to save time and get prepared for ze siriouz gameplay online.
Also, for the same price you can unlock "foil" version of the deck, which is the very same deck, only... Shiny.

Yup, there are multi-colour glitters on the cards, and I already have played with people using them (probably thinking that they're uber cool, and not realizing that they've been ripped off).
That's life - human population is so diverse that some folks' choices are guaranteed to leave you gob smacked- some people will buy anything that's available to buy for reasons known only to them.
Whatever DLC is, I can guarantee you that in some time (probably around the next expansion) old DLC's will be rolled-in available to buy cheaply of free with the next expansion. If you want them earlier, well then you'll have to pay stupid money.
My guess is that around CIV release they already had a lot of work done (or even finished) on Warlords expansion Civ leaders etc. So instead letting everybody wait a year til the next expansion they'll be giving bits of it as DLC. I don't care, I can survive waiting for Babylon
when I'll finally get my hands on the game. My goodness how hyped I am...