I've seen people complain about glass jars, and sometimes replaced with a plastic case without the owner's consent. Personally I feel the cache it'self is part of the experience. If it looks costume made or unique in any way it adds to the experience. I've bought 3 rely nice glass jars that I will hide somewhere where they won't break. I also bought a hand made wood chest, and I bet if people find it they will think there is a treasure in it. I haven't hidden my own cache yet, and I've only found 20 myself, out of these all have been plastic except one.
It might just be me, but I think there should be more variety then just plastic.
I do understand the pros in plastic, it's waterproof (usually) and durable.
Edited: I confused the words pros and cons