Yep . To me , the fish seem calmer and show their natural behavior best when kept only among their own kind . No other fish competing for food . No perceived predators making them nervous . Some find it boring but I have come to like species only aquariums. I do not believe in the myth of the community aquarium . But that’s just me . To each his own .
You know what. This is interesting timing. Over the past couple of days I have been asking myself that very question. For example, my initial plan when I returned to fishkeeping recently after a 10 year absence, was to have a corydoras only tank. And like you say, for some, they might have thought "strange, you will only have bottom dwellers". Which isn't quite true anyway, when I had a corydoras only tank years ago, the cory's were everywhere, up, middle, bottom (but of course, mostly bottom of the tank).
Anyway, I got my corydoras this time round, but added a large group of tetras and some young bristlenose. Now that mix, is not exactly far fetched, radical or controversial, but I now have been faced with issues such as the tetras eating most of the food before it reaches the corydoras and the clash with sinking tablets because the veggie based ones are good for the bristlenose, but not great for the corys. These issues are all overcome(able), but in hindsight, it adds extra aggro really.
I love corydoras, bristlenose and certain tetras. But I wouldn't do it again in the same tank. They all get on, but now it doesn't feel right. Like I am trying to have my cake and eat it.
At most I'd have two different species that offered no confliction. Not just the obvious hard water/soft water and temperature, but also things that we can take our eye off, like types of foods, methods of eating, lighting levels, water flow etc.
If I did have two species, it would not be an even split just to make it "look" balanced. I would have a dominating species such as corydoras, and maybe one or two single other fish that have zero conflicts like I said above. I certainly wouldn't have two "groups" of species, I think ever again at this point. I can really see your point.
For me, as long as their is interaction and interesting behaviour in your one and only big group (of one species) that will leave me content.