For the person who only runs 2 or 3 tanks of fuel through their saw each year, this is probably true. That person won't have the opportunity to sharpen often enough to get good at it. For the person who heats their home with firewood, it takes longer to get out of the vehicle, go inside, drop the chains off, and walk back to the vehicle than it does to actually just sharpen the chain. I've never been able to walk into a store of any kind and be back in the vehicle in less than 5 minutes. It's normally at least 10, especially if I have to speak to someone. Throw in the fact that you have to repeat the process to pick the chains back up again, and the fact that you have to take the chain off the saw, reinstall another one, and I'd estimate that even if you're rotating through 4 different chains, it's still faster to sharpen them yourself. Not to mention the fact that you get a better edge, and a heck of a lot more life out of your chain. Realistically, if you run your saw enough to warrant having 4 chains for the saw, then you're wasting a lot of time and money if you're not sharpening your own chains.
The other issue with your logic here is that you assume that the "reputable" dealer has a highly skilled technician who, after years of working through his/her apprenticeship program, has finally attained the job title of "professional chainsaw sharpener." Dealers make money selling new saws, and it's VERY rare they can afford to pay any of their technicians a decent wage. If they are lucky, they've stumbled across an older guy who's retired and who's looking for something to do. Even that guy doesn't want to spend all day sharpening chains because it aggravates the arthritis in his hands and wrists. For the overwhelming majority of the dealerships, they provide the service because it's expected of them, and it gets delegated to the lowest guy on the totem pole who would otherwise be pushing the broom.
To each their own. Not trying to tell anyone how they should do something. Just make sure you have all the information so you can make the best decision for you.