As someone mentioned, I would be concerned with signal degradation/loss due to metal features absorbing or redirecting the cell signals.
I work in an industrial environment, so I need some protection for my phone-- I use Otterboxes, and have to replace 'em about every five months or so (which is still cheaper than replacing a phone every six months).
Unlike most of my coworkers, I am not trying to peer through a cracked screen all the time, so plastic is not a problem for me, nor heft, since the Otterbox Defender almost doubles the thickness of my phone.