Why is laser illumation of a cockpit an emergency?
Let me posit a hypothetical game ("Don't Try This at... well Anywhere").
It's simple - there are just 3 rules:


  
  Give your friend a nice laser pointer and have them stand at the end of a road
  A half-mile or so should be plenty.  
  Get in your car at the other end of the road and drive toward your friend.  
  Your friend's goal is to shine the laser in your eyes. Your goal is simple: try not to crash.
  


Clearly this is not a very safe game.
It should be clear that playing the same game with aircraft is substantially less safe. Someone attempting to "play" without your consent certainly rises to the definition of "emergency" in my book.



If you actually play this game you'll note that the laser isn't a neat pinpoint when it's shining at you.
Between scratches in the aircraft's window and the way your eye perceives light the laser pointer can completely blind the victim - check out the examples in the graphic below.

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To date I don't believe any accidents have been attributed to laser illumination, but it is well within the realm of possibility, and the upward trend in laser illuminations of aircraft means the likelihood of such an accident is also increasing.

For more information check out the FAA's Laser Safety Initiative -- and particularly the video here.