Hey Guys,
I just had a chat with Motorola 2 minutes ago via the "Live Assistance" tool on their website. As you can see, I was purposely trying to ask the same question in a variety of ways to see her responses. I was trying to wear her down (lol). Also, I had to be patient as she would take several minutes to answer each question.
Anyway, for what it's worth, here's the chat transcript:

Denise C: Hi, my name is Denise C. How may I help you?

Me: Hello, I have a question about the Motorola Droid. There's a piece of plastic behind the screen. Is there a purpose for this plastic piece?

Me: This plastic is seen when you slide the keyboard out.

Denise C: I will be more than glad to assist you.

Denise C: Do you mean the plastic piece that it at the back of the phone when you slide it open?

Me: Yes, that is the piece I am referring to.

Me: The piece behind the screen when the keyboard is slid out.

Me: Hello?

Denise C: I am sorry for the delay.

Me: That's ok.

Denise C: The plastic piece behind the screen at the back of the phone is there to protect and to cover the screen.

Me: It's "behind" the screen, so how is it protecting the screen?

Denise C: It is to protect the back part of the screen where all the connections are.

Me: Which connections? So, that piece of plastic should not be removed?

Denise C: There are circuits and boards behind the screen that should be covered and protected from damages.

Me: I see, so for some users, their screens are getting "slide marks" on them and peeling off a bit. What is the down-side of peeling off the plastic?

Me: sorry

Me: i mean the plastic is getting marks on them, not the screen

Denise C: I see.

Denise C: Slide marks on this plastic cover are product of regular usage of the phone.

Me: So, basically, your recommendation is to leave the plastic cover alone.

Denise C: Yes, we recommend not removing the plastic piece behind the screen at any time.

Me: If it peels off through heavy usage, what do we do? Get a replacement?

Denise C: In case that you start noticing excessive or deep peeling off or slide marks on it, please contact us back so we can take of the warranty on the product.

Me: So, that means you would replace the entire phone?

Denise C: It might depend on all the warranty options available at the moment.

Me: Okay, thanks for your time.

Denise C: You are more than welcome.
"Circuits and boards behind the screen"? Kinda odd if you ask me...