Look at it this way - even if you needed all those maintenance items - which you probably don't - would you rather pay for them all up front even though some you won't need for 3 years? Or would you rather pay for them at the time you need them, if and when you need them?
This is nothing more than an insurance policy, just like extended warranties. If it was really cheaper for you (on average) the people selling the plans would go out of business. Sure, you can get lucky on one or two vehicles, but in the long run (key word long) you're better off insuring yourself.
I haven't bought any extended warranties on anything for the last 15 years at least. I've probably saved between $6K and $8K by not buying them, and I've only had to spend a few hundred on repairs or replacements that would have been covered. Right now I'm several thousand dollars ahead and even if I had to replace an engine or transmission tomorrow - I'm still ahead.