Depends on the motor too.
A 50hp+ engine is a LOT more expensive to have to repair than a 20hp is. The bigger the motor the more expensive it is to fix.
On the smaller stuff, it's up to you. What gives you peace of mind? The rebate or knowing you have more warranty if you ever need it.
Extended warranty is sorta like insurance. You hope you never need it, but if you do, it's nice knowing that you have it.
And on the flip side, look at the warranty statement. It doesn't cover everything.
We can't make up your mind for you. All we can do is give you the pro's and con's of each. I usually buy extended warranty; to this day I've had a claim for $18 and that was IT.
Now then....on extended warranty, manufacturers look at this when you have a "gray" claim-one that's not really warranty but it's not really your responsibility either. For instance, a paint issue on the cowling. You take it to dealer, dealer says hmm...I don't know if they'll help you or not. Ask them to call Yamaha and make them decide; after all they built it. In a case like that, the dealer is asking for a goodwill. If you have extended warranty, they're more likely to give you that goodwill than if you didn't have the extra warranty, and this principle is good for the length of the warranty. Worth mentioning, coming from dealer inside information, so to speak.