How tough is the aluminum body of a MacBook?
Solution 1:
The unibody construction is substantially more impact resistant than the previous plastic design. This is especially true in the display area where the front glass and the rear aluminum act more like an I beam than the old design. Not only is the construction more sturdy, but in falls, the damage tends to be less expensive to repair should you actually break things.
For example, the unibody macs with enclosure damage usually cost between $300 and $400 to repair where a cracked screen can cost upwards of $800 to $1200 for an Apple repair. The unibody parts require better tools and training to repair inside them, so that's about the only down side I can see to the newer design. Aluminum is really much stronger than the old LCD panel with thin metal braces and a plastic shell to cover the display. Even the new macs with matte displays are much stronger than the old pre-unibody displays on MacBook Pro / PowerMac line.
Of course, many single falls can break even the expensive parts in a unibody mac as the tolerances inside the machines leave little room for deformation to safely absorb the shock of a fall, but I have seen far more unibody macs that have skidded across pavement at highway speeds that still worked and had only surface damage than the previous generation of non-unibody macs. (Yes - it happens more than you might guess and when placed on the top of a car or truck, sadly they don't actually leave the vehicle until you get up to highway speeds. Now the ones run over by a following vehicle - unibody stands up better, but still doesn't usually survive that ordeal.)
Solution 2:
I have seen aluminum MacBooks with cracked screens, but I wouldn't say it's common and overall my observation is that the Apple laptops are as durable, often moreso, than anything other than an armored Toughbook.
Of course, that doesn't matter if you're the unlucky exception so whether or not to use a heavy duty case is up to you but I wouldn't be overly nervous. FWIW, I carry my own 2007 MacBook Pro–a more flexible chassis than today's unibody decks–naked (the computer, not the operator) within the padded Laptop compartment of my Tom Bihn or Patagonia bags.
Solution 3:
I had a MBP 13" go down in a loosely bundled rack attached to my bicycle. The MBP was enclosed in a Case Logic ~neoprene sleeve. The exposed corner of the sleeve hit the pavement and the friction of aluminum-neoprene-pavement tore out a hole the size of a pencil eraser at one of the corners. The MBP seemed to chiefly slide about 1 inch or so across the pavement and put a scrape at the battery-side of the aluminum unibody. All internal components function without any problems. Granted, this was not on the screen-side... however, I think the result would have been the same, namely, minor superficial scratching to the case. In summary, an arc-like fall from a 3 foot height, while cocooned in a neoprene case, caused no detectable functional problems. Incidentally, it probably helped that the fall was not 'hammer-style', but rather, the impact was of the MBP landing at a 45º angle.
Your mileage will vary. I do not recommend running with the MBP in its 'open' configuration. That's what caused my old HP to die a slow screen-related death.
Solution 4:
Out of maybe 100 Mac portables at my office over the last 3-4 years, I've only seen three that came back with significant drop damage, and only one of those caused display damage (and even then that was an intermittently blinking screen, rather than any cracks). I am much more concerned with liquid spill damage.
That said, the Speck hard cases do seem to offer additional protection.
Personally I carry my MBP 15" in the built-in padded sleeve of a Timbuk2 laptop messenger bag, and I bike with it fairly often..
Solution 5:
my macbook pro [ aluminum ] fell of the back of my ute onto the road going round a roundabout at 50 km hr and is still going year and a half later , it got a few dents in it but is still going trouble free