I thought you might be interested in a discussion of plastic mouthpieces in the tromboneforum.org site.
I have no opinion as I don't use them, nor have a need to, but others have found them to be valuable when playing in cold weather.
Here's the discussion site:
http://tromboneforum.org/index.php/topic,7491.0.html
It deals mainly with Kelly mouthpieces as they are the main manufacturers of plastic mouthpieces, but other alternatives are also discussed.
If you trombone players are not familiar with tromboneforum.org, I recommend it.
http://tromboneforum.org/index.php
There are over 4,000 trombonists registered, and most of them are upper level players, many pros, and the advice, stories and detailed knowledge of trombones is remarkable, informative and educational.

I use Doug Elliot mouthpieces with plastic rims, mainly because in the Navy we find ourselves playing out on the piers in cold weather waiting for ships to arrive and stuff like that. I've never found a need to switch to metal rims when playing indoors.
As for a completely plastic mouthpiece, Kelly doesn't make my size, so I never tried them. But knowing all to well the pain of playing in the cold with a freezing hunk of metal on your lips, I say go for it if you need to. Many of our trumpet players do.
As for our bones, a few of us use the Doug Elliot plastic rims, but none of us use completely plastic mouthpieces.

Yes, I've read several references to the Doug Eliot plastic rims. How in the world do they bond plastic to metal without loss of integrity?

It's not bonded at all...the rim screws on. You can switch back and forth from plastic to metal if you like.

Now how cool is that? LOL
If I had a reason for using plastic, that's the way to go.

do plastic rims make the feel of the mouthpiece more slick and not really sticky. I don't really know how else to explain it than that. I guess I'm saying does it make it more easy to slide around on it? I'm currently having problems with keeping my lips moist and unchapped, but everytime I use the mouthpieces though they get chapped and cracks.

Plastic rims feel more sticky than metal rims. If you don't want a mouthpiece to feel sticky on your lips, then DON'T use a plastic rim. It's great if you like a locked in feel. I use Doug Elliott mouthpieces too. To my ears it sounds like with a metal rim you get more resonance. Especially when I want to get that big, rich orchestral sound that really projects and rings. So I use metal rims most of the time. The only time that I use a plastic rim is when I play outdoors and the temperature isn't ideal.