Most likely, it is only to prevent dust from getting in there but it does cause some major problems. As shown below you can take out the plastic covering and it should significantly reduce some heating problems.
One thing to note is that this seems to only be for the MacBook. I checked my MacBook Pro and can't seem to find any type of plastic covering so I'm pretty sure this is a MacBook-only fix. Personally, I don't have too many heat problems but I don't have a Revision A MacBook Pro.