I travel with my 17 inch 2011 MBP all the time and physical protection is a necessity in my humble opinion - especially when in the vicinity of places like construction sites/steel mills.
All of my protective layers are from Moshi including the keyboard protector, palm guard/track guard and the Moshi Codex.
The keyboard protector is a must, especially if you're in a cafe where or confined location where one can be careless with liquids. Keep in mind that Applecare doesn't cover accidental damage. Even if it did, you'll be out for a week or more without your MBP. Is it really worth that kind of hassle because you or someone else tipped a coffee cup over your MBP because you didn't want to spend an extra $20?
As mentioned above, keyboard protectors are much easier to clean than the keys themselves. The keyboard keys can't be uplifted so if you get bits of food or other debris inside your keys, it's there permanently unless you dismantle your MBP.
The only outstanding issue with keyboard protectors is the effect of having an extra cushion on top of your keys which require additional pressure to depress the key. It's easy to miss key strokes when using a keyboard protector for the first time. Moshi's keyguard seems to be the least problematic in this regard but either way, don't expect to break WPM speed records.
Palm guard/track guard are nice to have but optional if you're worried about palm grease which has the potential to discolourize your MBP.
The Moshi Codex is one of the few sleeves available for the 17 inch. Not only is it well manufactured, but designed so that the sleeve can be worn even while the MBP is in use. When the sleeve is unzipped, the zippers are laid flat as to allow access to all side ports. The bottom of the sleeve has supports that raise the MBP about 5mm or so off the surface for heat to dissipate. The extra bonus of having the sleeve on while the MBP is in use is that it covers the lid casing, concealing the Apple logo so as not to draw attention to potential thieves looking to snag a Macbook. Lastly, the Codex has a nice fold out handle so you can carry the MBP like a briefcase instead of carrying it under your arm as it were a heavy textbook.
Of course, the codex does an excellent job of protecting your MBP from scratches/scuffs and the elements - which can only help retain its resale value.
If there were any disadvantages with the Codex, I would have to say that it does increase the thickness of your MBP slightly which can make it tricky to insert your MBP into a bag's laptop compartment.
I don't use a hard shell myself due to the fact that I carry my MBP in my backpack so generally I'm not too worried about dropping it.
The bottom line is that all these guards and casings not only offer practical protection that will save your time and money where minor accidents are concerned, but they're many more times easier to clean than your MBP and are therefore worth the extra expense.