Does always using Force Quit to close programs make problems w/computer?

A friend was over and we were working on my iMac, and I used force quit to exit a couple of programs - and he gasped and said "Don't do that! It will cause problems with your machine!"

He sort of said something about directories - but ultimately I didn't understand why using force quit isn't the same as just closing, or why it would hurt my computer... Does anyone know?... My machine is running slow and has some quirks and I need to do a clean install because a system reinstall didn't help much. But I've also been downloading demos and trials right and left since I got it - which I've just found out is not a great idea and I've found out how to fix it from another forum.

So I don't know if what my friend says is true because I know I've done so many other weird things to my poor iMac out of sheer ignorance that it could be anything......

Is he right? Why wouldn't they give us that warning?

thanks -

maggie