Cranky old cyclists may be optional, but crank arms aren't. 

Returning from a ride on one of my usual routes today, I heard the siren of an emergency vehicle approaching from the rear and looked for a place ahead where I could pull off the road.  As I unclipped my left foot to prepare for landing, it felt like the chain had slipped off the cogs.  Looking down I noticed that the left crank arm had separated from the bottom bracket and was dangling from the pedal still attached to my cycling shoe.  We Engineers have a specially developed sense that tingles when something is amiss (sort of like Spiderman's spider-sense that warns him of danger), and my Engineer-sense was on full alert telling me that this wasn't normal.   After the fire truck passed by (the whole world is on fire these days), I limped ahead to a favorable spot to deal with this interruption of my cycling bliss. 

The crank arm is a Shimano Deore that attaches with 2 pinch bolts.  Fortunately it's an integrated crank system with externally mounted bearings, so I never lost my bearings - not only did I always know where I was, I also didn't loose any of those steel marbles.  The fine print on the crank notes that each bolt should be torqued to 12-14 newton-meters, so I closed my eyes and let my Engineer-sense guide me to the exact torque required from the 5 mm allen wrench of my multi-tool.  The entire repair only took a few minutes, and half of that time was me laughing at myself; "Dude, WTF?  The crank arm came off while you were riding, you moron!" 

I admit that I'm a bit lackadaisical when it comes to routine bike maintenance (perhaps not as bad as our own Greg Garceau, but obviously I have some inclination to just ride until things fall apart).  I do check things over once in a while, lube the chain and check it for wear, replace brake pads when needed, etc.  But apparently I haven't paid attention to the tightness of the 2 bolts that hold the left crank onto the integrated crankset.  Lesson learned, or so I hope.    

I'm sure you are a long way off from my level of bike neglect, yet I'd like to think I'd notice a loose crank arm before it came off while riding.  I've gotten several chuckles from my mental picture of you standing there with a crank attached to nothing but your shoe.

As I was repairing the crank, I remembered old black-and-white episodes of the Three Stooges driving along and the steering wheel coming off. Fortunately my handlebars remained attached, but I think I will check them before my next ride.

Also, you have to pay attention to the little plastic doohickey (yes that is the proper Engineering Term for it) in the slot between the pinch bolts.  If it isn't fully seated the arm will be liable to fall off, even if the pinch bolts are properly tightened to the specified torque.

I think I got it seated properly - it was a beside-the-road repair and the plastic thingy was protruding inward at first, which prevented me from being able to seat the crank, but once I put in back in position the crank slipped on. So hopefully I got it right. I did 45 miles today and everything worked fine, nothing else fell off the bike (as far as I know anyway).

For the benefit of anyone not already familiar with the Plastic Doohickey, here are a couple photos.

I've seen a YouTube video that claims that failure to re-seat the Plastic Doohickey can result in the arm falling off, even when the pinch bolts have been properly torqued.  I have not experienced that myself but then again I've only had that crank apart once in the 15 years I've owned the bike it's on.

What's the advantage of a splined crank and doohickey compared to an old fashioned square taper crank that doesn't fall off?  ;-)

AFAIK the bearings are closer to the cranks which is supposed to be a good thing for those strong riders with quads like tree trunks but I agree that the old square taper technology is a lot easier to look after, just check with an 8mm hex key when ever you clean your bike and the cranks are unlikely to fall off!

The snarky answer is that it helps ShimaNO sell more stuff, maybe?  They're notorious for issuing new designs that are wholly incompatible with all previous models and discontinuing said older models to force you into buying all new rather than repairing still-serviceable older stuff.

But truthfully, I really have no idea.  That's what came with the bike, I didn't choose it.  However, I will say that bottom bracket maintenance is reduced basically to zero.  Could that have been accomplished without changing the spline design?  Not for me to say, because I dunno.