The real issue with the xkcd password is the fact that because of the comic's popularity, it is now common knowledge, making it about as secure as "password".
As for the method used to create the password, your claim that it's vulnerable to "a clever dictionary attack" is misleading. To crack the password in a few days, like you suggested, the attacker would have to:
- know the method used to generate the password
- know the exact list of words used to generate the password
- have access to the hashed password (instead of, for example, attacking a server)
Finally, the point of this method is to create a password that is hard to crack, but very easy to remember, and it achieves that.