Well for once i rely on M$ for making the decision on.
"when to defrag"
It tells you if you go to defrag.
Or there are defragers out there that just run automatically in the background when your system is at idle.
Not so sure about the HD wearing out, suppose it could do, but what are the chances of having a HD for an extended period of time, i still have my very first HD, i got it when Win 95 came out, so its over ten years old, still got all the stuff it used to have on it, still works, etc.
Since then i have had well over a handful of systems, just backup my HD with each one i get.