Why do we need Vanilla/TBC/WOTLK?

If Cata comes out, wouldn't we just need Cata's CD key only? Or do we need the previous 3 CD keys?

Yeah but we can just get the previous installers from friend or cybercafes.

Yalor, just buy Cataclysm for the CD Key.

The others just pinjam the installers from friends, cos we don't need their CD Key? Correct me if I'm wrong hmm.

Okay okay, maybe I do this step by step, see if this works or not. Sorry for my noobness

1. I borrow WoW Vanilla installer CD from friend. Install game in my PC.

2. I borrow TBC installer CD from friend. Install expansion in my PC.

3. I borrow WOTLK installer CD from friend. Install expansion in my PC.

4. I buy Cataclysm from store. Install expansion in my PC.

5. Using Cataclysm CD Key, I activate game in Battle Net.

6. Play

Can or not?

Ah okay... thanks!