Polycarbonate is light, strong, doesn't interfere with antennas like metal bodies sometimes can, and flexes and recovers when dropped, unlike metal bodies which often dent. It's also much less expensive to mold into whatever shape is desired than a full metal shell.
I'm much happier carrying the plastic S3 in my pocket than even my smaller, but heavier, HTC Evo 4G. I have no desire to carry a phone as heavy as an all metal version of the GS3 would be.
I think the love for metal phones come mostly from people who need to find some reason to dislike Samsung phones that are, in almost every respect, better than their favored brand. It's the same sort of snob appeal that leads people to brag about their BMW or Mercedes when, for their driving style, a Honda Accord would be more reliable, more comfortable and more economical. The Honda (Samsung) just doesn't look as expensive sitting the driveway (your hand), so you can't impress the neighbors as easily.