MacBook Pro keyboard cover

Just got new 17" MBP (last mac purchase was a blue and white G3 which is still going strong, just can't run CS), and spousal unit suggested a silicon keyboard cover (she swears by them on her Sony VAIO). It showed up in the mail the day after the 'puter did, and I'm reading the "hello" booklet that came with the MBP and it says "do not put anything over the keyboard." Is the a warranty no-no? Is the protection from dirt, dust and possible liquid spills out-weighed by whatever reason Apple has for the ban? I know they look at liquids damage as "abuse" so is this a revenue generating stream? Does the MBP cool itself around the keys? any thoughts?
Thanks!

The keyboard area provides air intake for the cooling system. While there are other intakes, it's not advisable to cover one of the largest and stress the system.

As for possible dust and dirt -- I would exercise general maintenance like wiping the exterior with a damp (not wet) cloth regularly to lift off any major particles before they get sucked inside. And although it's rarely necessary, if you're somewhat adventurous, as I am, you can open the case every year or so and gently blow away any accumulated dust with a can of compressed air (don't use your vacuum).

As for liquid spills -- don't drink around electronics.

You can, if you like, use a silicon (or whatever) keyboard cover on your MBP when it's not on or is asleep, of course, as no heat/ventilation issues come into play. I use one on my wireless keyboard as my skin is very oily so I don't want to transfer that oil onto the keys.