Is this true? Plastic covers seemed thinner than the glass/metal shells that have replaced them. Also, from my limited experience, the batteries in the glued-together phones have adhesive strips that secure them inside the case, which again add a little extra thickness.
I could be wrong about those things, but I stand by my assertion that the plastic snap-fit phones were more durable. Durable enough that they didn't need cases for protection, which above all else rob a phone of its thinness/sleekness.
The first time I dropped a glued-together phone, I cracked the screen. I thought it must have been a fluke, since I'd dropped plastic phones tons and they'd always been fine. I was so sure it was a weird one-off I refused to get a case after having the screen replaced. My girlfriend called me an idiot. Two months later, I dropped the phone again. Now I have a case.