Do you think learning Japanese for the sole purpose of playing Japanese games to be pretty stupid?

No, to each his own.

No! Any time your learning a new thing your making new neural connections in your brain. More connections = more brain power and a better chance of keeping all your cognitive functions as you get older.

Yes and no. If it's what you want to do, go for it, but if you're going to devote so much time and effort into something, it should pay off more than just for gaming.

Rather than stupid or silly, I'd call it passionate. You know it's hard and unusual but you have your mind and heart fixed for it. Personally I like persons like that. I think they're cool.

No ! Cultural Expansion is quite diverse its good to learn new things however I hate to say it you will most likely end up forgetting it if you do not use it for a while

I guess because japaese is really hard