In my opinion the only good thing it does is to create copies of already existing communities, so basically people tend to follow one community or the other and it divides the people who could be active.
If you want to create a community that’s similar to one and for some reason you don’t want to be a part of it, find another name, if you can’t find another name, create your own instance. So if Lemmy is federating now we could have /c/worldnews, /c/world_news, and the same applies to every other instance that decides to do the same. In my opinion this only segregates people.
The same applies to uppercase letters, which Reddit uses but luckily Lemmy does not, imagine how many copies of a community could be created if you use both.
I mean… I think there are a couple of valid examples, I think it would make more sense to create a community named /c/chickencity and there you’d ask for places to eat, though. But let’s say it’s a valid example, in my opinion the pros are more than the cons. How many copies of communities can be created by adding the _ and how many communities will need to phrase their address in a different way because of it?
I don’t know tbh. You can make two copies of essentially the same community around the same interests/subjects with different names, underscore or no underscore. I think “mechanically” solving a problem like that is not a great idea, and that defining if two community should be merge fall into the hands of admins and administrators, not developers. If you are a mod of a community, or admin of an instance, chances are you know that such “clones” exist, and you either think you should merge and do it, or think you have a valid reason to stay apart and are free to do so.
Of course you can make similar copies, the thing is that with this issue people create them without realizing it and it super common, whereas having to find a similar name it’s done on purpose.