Is it a bad idea to leave your home pc running all the time?

My husband leaves his pc on for DAYS at a time and I am wondering if this is harmful to the life of the pc and it's components. It's just on for the sake of being on, he's not downloading/uploading/running programs. I'm concerned that he may be shortening the life expectancy of an otherwise perfectly healthy, virus free pc...lol. Please tell me what damage if any, is done by running home computers for DAYS and even WEEKS at a time with no shutdown.