You can't think of any reason that a call log longer than a couple of weeks could be practical?
Even in your own example, what if you didn't foresee having to call that person back 6 months later and didn't make a contact for them?
For work reasons, someone may need to show that they had a conversation/ attempted numerous calls and getting the information from a phone company (especially on a work cell phone) isn't always the easiest thing to do.
If I don't think I'll be needing a phone number again in the future, I generally don't make a contact for it to save to. In the past (with other cell phones), it's been a nice blessing to have that number listed in the history... or, on some phones, it would automatically pop up if I began dialing.
As with most everything else, just because you don't see a need or have a use for something, it doesn't mean that others don't.