I don't see what the big deal is about plastic... seems less fragile than glass, no? Anyway, I can't speak for anyone else, but I bought a Samsung phone because I wanted a Nexus phone, and the Samsung Galaxy Nexus was the latest one, so that's the one I got. If I'm eligible for an upgrade and the Nexus phone of that time is with another manufacturer, I'll get that Nexus phone. No loyalty to Samsung here.
That said, if I had someone ask me for an Android phone recommendation, it's very convenient that Samsung makes a high-end Android phone (that gets good reviews) that's the same phone available on all four major US carriers (Verizon, AT&T, Sprint, T-Mobile), so I'd probably recommend the Galaxy S III.