I bought an extended warranty on a bike once, a Honda ST1300. I knew Hondas generally don't break down but since the bike was both quite complex and I used it for long distance travel (3-6,000 trips mostly) I decided to try it. Last summer two weeks before a trip to the west coast I went it to start the bike. It would run for 15 seconds and the die no matter what I did. I called Honda. They came out and towed it to the nearest dealer, 'nice. What was not so nice was the injectors, wiring harness, and control unit that the mice had chewed into and thus taken out, nearly 2 grand worth of damage, none of which was covered by Honda. Now, was that really Honda's fault? No, I bought basically something that deferred the risk of manufacturing and design deficiencies after the stock warranty period. The thing is, bikes like Honda's and Yamaha's don't tend to have much in the way of deficiencies, particularly after the enitial warranty period. Had I not bought the extended warranty I could have used those dollars to cover the repairs. Now, I have a borderline psychotic cat and a bike with a lot fewer places for vermin to set up residency (S10). If I were ever to buy something with a known history of expensive manufacturing-based problems (say a BMW or Ducati) I might pop for a warrenty extension again but unless you abuse your bike regularly, I think the odds of you breaking even on an extended warenty for your basic Yamaha or Honda are pretty low. Remember, they are not covering problems with the bike, they are covering problems related to origional design or maufacturing issues. That's a pretty low percentage and thus a fairly long bet, YMMV.
