Easy answer: You and the rest of the world should always buy it.
Semi-Easy answer: You should buy it but realize it is not insurance and only will cover certain things. If what it does not cover is something you have a real-time worry about, purchase insurance, or add it to an existing policy as well. Also, still buy AppleCare.
Long answer:
-AppleCare covers the 2nd and 3rd year of a laptop's life in which statistically speaking, at least 35% of all laptops will have some type of an issue.
-That 35% is probably higher due to people not making warranty claims on certain issues
-All laptops, including the MBP, have more parts in it than Einstein could have contemplated and many of these parts can malfunction
-Statistically speaking, if you have 1 issue over the 2 year period, you will have probably saved money with the AppleCare due to most issues that the MBP has cost hundred of dollars to replace/repair
-Many problems require multiple repairs
-If you have AppleCare, you will not have to live with certain annoying issues (ex: dull screen, unlit keys, power cord fitment issues, etc.) PROVIDED you actually call them...some people live with them and complain about them where as a simple call to Apple would fix it within a week
-Apple often WILL replace batteries that have decreased capacity due to NORMAL wear for people with AppleCare ALTHOUGH they are NOT required to. You can find a lot of reports about this
-Statistically speaking, items with an active warranty sell for more money and tend to sell faster than identical items that are out of warranty.