A keyboard condom is too anal-compulsive for my taste, and I am the type of person who tends to go to significant lengths to protect my Macs... I have used a variety of portable Macs during the past 15 years and I have never had a keyboard problem caused my dirt.
Computers are meant to be used; they are not rare museum artifacts. They will pick up some dirt and scratches along the way but if you take reasonably good care of a computer grime will be kept to a minimum. I just sold a 3 year old 15" MBP and it was in mint condition. All it took to maintain its good looks was regular cleaning with OmniCleanz, a Radtech keyboard cover and a nice sleevecase when away from my office.