Hey There!
In the scenario you've described, my answer would be "no", creating a landing page for a single location brick-and-mortar business does not make good horse sense. As you've rightly pointed out, the entire website is optimized for this location, nullifying the need for additional landing pages. If the dental practice opens a second location, then yes, do create these types of pages, but right now, it's not necessary either for patients or for your SEO efforts.
More on this: right now, the dental practice's only association with cities other than its city of location is that some patients may come to the locale from additional towns. That's fine, but it's nothing to really publicize. After all, a famous restaurant might have diners travel from all over the world to patronize it, but it just wouldn't make sense for the restaurant to build a landing page for each possible customer origin, right?
So, if the dental practice wants to build awareness of itself in neighboring communities, then it needs to have some stronger relationship to publicize than the fact that some if its patients live in towns B, C or D. This is where the role of relationship building comes into play. A dentist in Town A could possibly sponsor events in Town B, or speak at a conference in Town C, or offer a discount to parents for kids' first dental exams in Town D. These would be actual topics to write about on the website, or to promote via social media. Genuine relationships could be built by this type of outreach, and while it wouldn't be highlighted via landing pages, it could begin to build up brand awareness in these additional communities where the dentist lacks a physical office.
I'd also advise the dentist to consider what he can offer, in Town A, that is lacking in Towns B & C. For example, I traveled several towns away to a holistic dentist to have my mercury filling safely replaced for health reasons. No one in my own town offered this. If I were that dentist, I'd be doing everything I could to publicize rare or valuable services that are worth traveling for.
And, in the meantime, be sure the Contact page for the single location rates an A+ in thoroughness.
Hope this helps!