The only time I had a service contract was when the dealer threw it in for free--because we could not take a sea trial (Lake Michigan had frozen up virtually overnight). It was an 8 year Mercruiser warantee--and sounded great. I had some outdrive problems (hit a small stick with a bravo III unit, destroying the gears)--and the warantee didn't do anything. My insurance did pay for the new outdrive. I sold the boat with 4 years on the contract and it was transfered. The next owner had some problems, and although they varified that the transfer was legal, they did not pay, saying that there was a maintance issue which was the cause.
I do not feel that the service contract is worth the money--unless it is free.
Modern outboards are very reliable--some are comming with 6 or more year warantees, and lasting thousands of hours. I would not buy it for a Honda 40--but you have to use your judgement and balance the potential risks of a probelm vs the cost of repairs. The Honda is a great engine, but be sure and wash it out each time if used in salt water.