It's also important to consider "regular usage" - the iPhone NEEDS a glass screen, because it's constantly being touched by the user. A plastic screen will NOT survive long with this type of usage, because it will just get scratched to hell quickly. After 6 months of having my 3GS naked, the screen had maybe one scratch on it from a drop, but the back was COVERED in tiny scratches, and I wasn't constantly dragging my fingertips around the back. I also had a Blackberry Pearl for a year and a half, and the plastic screen covering got so badly scratched from standard use that I could barely use it towards the end, and it just looked trashy, much like the back of the 3GS after 6 months.
A glass front and back makes for a much more scratch-resistant phone - this is a good thing for increased longevity! Yes, it's also more prone to shattering if dropped, but really, the back comes off with only 2 screws, making it VERY simple to replace, so that's kindof a non-factor, and when you remove the back glass panel from discussion, this puts us back at square 1 - the front screen HAS to be glass, which is why pretty much all touchscreen smartphones have glass screen. Furthermore, I don't think the front glass actually is more likely to break than on the 3GS - if anything, less likely, as it's made from tougher glass. Sure, I do have scratches on my IP4, but far fewer than my 3GS at this point... I've also just recently (less than 5 days ago) put a Skinobi skin on it, and I'm very happy with that so far. We'll see if it lasts.
There is also the matter of statistical noise and the simple fact that it's poor logic to draw conclusions about causation from correlation, as has been mentioned before.
A glass front and back makes for a much more scratch-resistant phone - this is a good thing for increased longevity! Yes, it's also more prone to shattering if dropped, but really, the back comes off with only 2 screws, making it VERY simple to replace, so that's kindof a non-factor, and when you remove the back glass panel from discussion, this puts us back at square 1 - the front screen HAS to be glass, which is why pretty much all touchscreen smartphones have glass screen. Furthermore, I don't think the front glass actually is more likely to break than on the 3GS - if anything, less likely, as it's made from tougher glass. Sure, I do have scratches on my IP4, but far fewer than my 3GS at this point... I've also just recently (less than 5 days ago) put a Skinobi skin on it, and I'm very happy with that so far. We'll see if it lasts.
There is also the matter of statistical noise and the simple fact that it's poor logic to draw conclusions about causation from correlation, as has been mentioned before.