Computers are not perfect. Everyone, even Mac users (arguably the system that spends the most time thinking through ways to make the system run without a lot of administrative overhead), spends some time troubleshooting problems.
John Gruber hits on the relation of this to Apple disallowing background apps on the iPhone.
Imagine a scenario where background apps are allowed on the iPhone this summer. Some typical user buys and installs 10 apps from the App Store. Three of them are background-capable apps, and two of those three are so resource hungry that they have a noticeable drag on battery life. How are typical users ... supposed to know which apps are causing the problem? How are they even going to know which apps do continue to run in the background? They won’t. A likely reaction would simply be to regret ever having junked up their iPhone with any third-party apps at all.
I'd go even further. Us nerds are all debuggers at heart, and I personally still don't want to spend too much time fixing my phone. "Oh sorry Joe, I missed your call 'cause I had to reinstall iPhone OS on my phone." The iPhone's claim to fame is it's usability not it's features. If the administrative overhead of owning one (meaning the average user has to spend any time worrying about which apps hurt performance and which don't) starts to cut into it's usage, the usability advantage evaporates.
People (and by people I mean average users) are willing to accept (or more likely have been conditioned to accept) administrative overhead on their computers. OS reinstalls, re-installing applications, fixing registry settings, calling support, etc.. They are not so tolerant of their phones. Phones out there may have terrible, terrible user interfaces, but no one spends any time worrying about administering them. If Apple wants to make the iPhone as ubiquitous as the iPod (which I believe they do), they'll need to keep that overhead low.
Apple would be lucky if a user that junked up their phone and caused system/performance problems blamed the applications they installed. More likely the thoughts on people (meaning regular, non-nerd people) would be "this thing crashes all the time," or "man battery life sucks." And they either return it, or leave the platform when the phone wears out, thinking to themselves "that's no better than any other phone."
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