To a point you are correct. If a PC is JUST used for business type applications then yes...it should always run happily.
BUT..in a home..they are not. Just based on "day to day" running, doing all the things that modern PC's are able to do, your operating system WILL suffer. It just depends to what degree.
I would almost guarantee you that as soon as you start surfin' "programs" attach themselves to you system. Odds are that you wouldn't even know, all seems fine for the time being, perhaps for ages, but stuff is plonked on your PC without you having a clue.
Spybot is a great little program to seek some of them out. I'll clean the PC using Spybot, will surf some completly "legit" sites and 10 mins later, Spybot will find some back again. Most are harmless BUT some will slowly strangle your system....and some my even kill yer PC completely. As I say, XP seemed to be especially prone. Windows 7 seems to have them licked.
Nowadays PC's are true multi-media devices and in our case, a "window to the world" for a whole host of reasons...Business, games, movies, music, you name it..it does the lot.
As a result, "crap" just "sneaks" onto them, some obvious, some not, and slowly, over time they can grind to a halt. I found XP to be the worst. Vista was better on a desktop machine BUT Windows 7 is the business..Love it !