It's basically insurance -- and the only time it really makes economic sense is at purchase, when a 3-year extension is about $100. After a point (4 years with Dell), shorter with Apple and others -- you can no longer extend the warranty, so if your system is now about four years old, there's no further extension to purchase (and at that point, any funds are better put toward a replacement system than into repair of an old one).  
Contacted the Dell Support maybe 2 months ago and never heard back. Do they actually care about customers wanting to extend their warranty? I have 1 month left to do so and it was actually very useful when I had an issue with a part on my laptop.
Extending Warranty?