My dealer explained the warranty and advised that I make the decision after a year or so, when I know if I want to keep the bike long term. In his view the major mechanicals are bullet proof, but UK conditions are harsh if you ride year round. I'd had problems on my previous bike (rectifier and oil consumption), so was tending towards the peace of mind, as it was relatively cheap. I would have bought it there and then, but decided to wait and see. Spent the money on a grab rail and centre stand instead
Downside is if you want to tune the bike, ECUs exhausts etc. Or mess around with electrics too much, you risk invalidating it and 4 years is a long time... (I guess unless the dealer does the stuff for you).