We actually just wrote an article on this last month, as we have been getting a few new clients that had really spammy tactics on their site that needed to be cleaned up.
If you search Google for "doorway pages vs landing pages," you'll see the article (or click my signature and go to blogs).
Anyway, one of the key sections from the article is this: "A doorway-style location page usually looks like someone copied the same page 25 times and swapped the city name. If you can place two city pages side by side and 90 percent of the content is identical, that is a red flag."
If it's not unique content, it's not worth having. If you create the page solely for ranking, it's not worth having. Let's say you did make the page for ranking, and it ranks high, and now a potential customer lands on the page - are you presenting them with generic gibberish, or are you providing them with anything useful that will convert them to a customer?
For us, we usually suggest clients have information about their service, relatable city info (just like
@fisicx mentioned), and a testimonial from someone in that city/region. We work with a pest control company, and one of the things we encouraged them to do was take a picture of their truck near landmarks, or "welcome to XXX town" signs. If they work with recognized local businesses, then a picture outside those too.
The crazy thing is, you likely don't need dozens of pages for each service in each city.
Another good test is to think of it this way;
If you replaced your Service Page in your navigation with one of your city-targeted pages, would it still make sense to the user? If so, then keep it; if not, dump it.