I think people just set those times so they DO have enough time IF they need it.  Alot of times, I have decided what I'm going to do 3 or 4 in advance.  Also, sometimes you only have one move.  Your point isn't really controversial, so I don't know if I can concur, but I think I do. 

Not always true, for example when I played games for the England Live Chess League I used up most of my (roughly) 30 minutes.

No one is stopping you from using all of your time.

It should give you an advantage