You know, I'm a big user of straight punches, but sometimes when you see guys use looping punches like Chuck, I think it's actually a sign of a more advanced striker.
Put it this way. If everyone you fought was a complete idiot or beginner, you would only need to use straight punches. The purpose of the hook is to get around the guard from the straight punches, and the uppercut is best used when the hook is being blocked at close quarters.
When guys are really good at defending the basic boxing arsenal, it's the looping punches and overhands that start messing with them. I'm sure Chuck Liddell, in all his years of martial arts, can throw the basic punches of boxing. The looping ones just keep guys on their toes, and hey, they work for him, so why not use them?
I used to get frustrated with the stand-up in MMA, because I came from a background in karate and Krav Maga, and I learned boxing, but I never had an "alive" background in stand-up. (I do wrestle though) So when I saw a lack of form in the Octagon I though it was because they were bad strikers. Actually it's because sparring and fighting is not as pretty as shadowboxing and the heavybag.