The answer to this depends almost entirely on the mechanical aptitude of the user, and not the equipment itself. Some users cannot (or will not) even so much as adjust a carb. My neighbor is a good example, he will run his blower every week for 3 years at 2/3 power and he's oblivious to the fact that it isn't running correctly. If you remind him he may clean out the air filter once in a while but he has absolutely no aptitude for small engines and couldn't identify a properly running one vs. a poorly running one until it quits on him. For him, a service plan is pretty much a given.
On the other hand, I would never buy a service plan on any piece of equipment regardless of cost. I enjoy operating and maintaining small equipment and can tell by listening if an engine is running correctly. I can tell when my neighbor's equipment is running correctly. I have no problem stopping in the middle of a job to tweak a piece of equipment if it isn't running perfectly. Or at least I will make a mental note and throw it on my workbench that evening or that weekend. I'm not going to turn my valuable equipment over to some kid at the mower shop to strip screws and charge me lots of money for guessing at problems and swapping parts.
One other point- I will not buy any brand of equipment if I believe it is going to break on me in the first year or two. Maintenance and minor items I'll handle myself, major items will be covered under the manufacturer's warranty anyway.
So the answer depends on how you define yourself rather than what equipment you own or how much the service plan costs.