I actually started on Kbin.social, but then it got shut down, Kbin died and now fedia.io seems to be the largest one running MBin. I like the interface on MBin and I guess it's good to have a diverse fediverse with different services, but at the same time, why use mbin when everyone congregates on lemmy instances? The local magazines on fedia are for the most part, quite dead, when compared to lemmy collections. In the end I feel like there aren't enough people to go around to support many more services like MBin and Piefed.
Simply by choosing a lesser used fedi software you're helping keep the fediverse from being dictated by a single software's whims. So that's a big plus there. Federation issues with kbin/mbin/azorius/other lesser used instance software will inevitably happen as people only test against the largest player in the field (in the ""threadiverse"" that's Lemmy, in the microblogging fedi that's Mastodon). So simply by not picking the largest you're, even if in a small way, helping not only mbin but all the lesser used fedi software as a whole.
Your own local communities being "dead" mainly boils down to communities themselves having a network effect around them where the largest one keeps growing larger as everyone focuses on it. And the largest communities are usually on lemmy.world (or occasionally other Lemmy instances). There isn't that much you can do there.
In my experience, it's always the smaller software that innovate. The same is true in the microblogging fedi (emoji reactions, quote posts, markdown, nomadic identity, reply permissions) just as it's true in the ""threadiverse"" (combining communities together, the ability to follow people, polls apparently (?)).
So really, don't worry about the size of your own instance's communities. As long as you trust your instance's staff to keep you safe there's no real reason not to get on a smaller instance, or on different software. Especially on here, where "discoverability" is not as much of an issue as it is in the microblogging fedi.
I've registered on a bunch of Lemmy instances, but I stuck with lemmy.ml because for the most part it seemed to have the least amount of downtime IME. Though I think they're all pretty stable these days anyway.
What you said is very true though, it helps to try out and especially actively use alternatives, since that's how you end up with software like Chrome (not really but kind of, you get the idea).