The arguments are:
1. Plastic is more oxygen permiable than glass. For the relatively short amount of time our beers are in either, I suspect this is mostly a non-issue.
2. Plastic scratches and the scratches harbor bacteria or other nasties. True: plastic gear will eventually need retiring whereas glass carboys theoretically last forever.
3. Glass is see-through. Although I normally do my primaries in plastic, I have to admit that glass primaries are an awful lot of fun because it's so cool to watch what's going on in there. The advantage for secondaries is a little more tangible, IMHO. Since the primary purpose of a secontary fermenter is clarification, it's definitely easy to tell when that clarification has been achieved in a glass carboy than in an opaque or transluscent plastic bucket.
So, yeah, there are some tangible advatages to glass, but I think it's largely style. Glass is also breakable, more work to transfer from one container to another, and it's really hard to get an ounce of whole hops into a carboy.
Bottle conditioning: Robust Porter
Drinking: Saison Dupont clone, tripel
Coming soon: Columbus APA, Rich Red ale