For my 17" 2.3 MBP, I picked up AppleCare mostly because I shelled out $2500 on it and would like to keep it protected for some time. Plus, the AppleCare also covers my Thunderbolt Display, so $3500 of equipment getting covered is worth it here.
For my old 13" MBP, heck no! The thing is barely over $1000, if it breaks and is beyond economic repair, I could probably recover some portion of the costs just selling it for parts. And besides, Amex will cover up to $1000 in warranty coverage, so there's no real benefit for AppleCare on the 13 MBP.