UML Diagrams are a lot more important then you would think. Sure you can go out and make the game then reverse engineer them but you will run into problems along the way.
The goal for us when we create UML diagrams is to plan out how the code is going to work and interact so we can find some of the problems before they occur. And hell it only takes a week or two. (maybe shorter if your only making a small game)
At first I despised them, then I used them in a project and they made everything so much easier. If you get used to making them then you will love yourself for it because you will start using them as your suedo code.