If the bike is running well, I'd guess the plugs are fine. Ducati is not the most together company when it comes to this stuff, but in my experience, what the dealer is saying is right, if not about policy, then about the plugs. I'd trust the dealer more than the "guy in an office". The dealer makes their money on repairs. My plugs were fine. I do this basic maintenance myself and after 18k they were fine.
Removing the tank the first time is a pain - after that it's 20 minutes max. ( a briefer pain). I think it's good to do the basic maintenance yourself because you'll do it carefully, and you'll look more carefully at the bike. For example - while doing unrelated work this winter, I found a hose clamp that had loosened, and I was losing coolant.
I also found a misrouted hose, that over time, could have worn on a sharp metal edge. It was just a case of pulling it into place, but over a few thousand miles, it could have left me stranded. My guess is the mechanic moved it out of the way to do the valves, and forgot about it.
I live in a city, so I do this stuff without a garage - it is doable. i rode with a guy who would wipe his bike down every morning - not because he was tidy, but because it made him look at all the bits and pieces in case something was obviously awry.
The more hands on the better. That's my POV.