The golden rule of programming style
There's an interesting page on the subject of compilation units per file over at the scala style guide.
The guideline, is, delightfully vague, which I will paraphrase as: Mostly use single files, unless you can't, or unless it's better if you don't.
The author(s) go on to expand on the reasoning behind breaking the guideline:
Another case is when multiple classes logically form a single, cohesive group, sharing concepts to the point where maintenance is greatly served by containing them within a single file. These situations are harder to predict… Generally speaking, if it is easier to perform long-term maintenance and development on several units in a single file rather than spread across multiple, then such an organizational strategy should be preferred for these classes.
This touches on what I consider to be the golden rule of programming style: Make your intent clear and the code easy to read.
Software spends most of its life in maintenance, which is why we have style guides and coding standards. It's valuable to have consistent looking code to promote a shared vocabulary, improve readability, and steer away from confusing or error-prone constructs.
It is just as important to be able to understand, both as an author and as a reviewer, that in certain cases following the letter of the law goes against the main goal of improving readability and maintenance. A one-size-fits-all rule does not always work, and as the authors of this particular guideline mention, "these situations are harder to predict."