It's worth considering the different targets of each port too.
Lemmix: This was originally created as a windows-compatible version of DOS Lemmings, and intends to mimic it exactly (and does a damn good job of it), except with some added fine-control features. (In fact, I believe that project was initially done as a playtest mode for a level editor, then was later built into a standalone player when the author realised "why not?")
NeoLemmix: This was created to be based off the same interface and feel as Lemmix (and by extension, the original DOS Lemmings), while removing the glitches that break levels, and adding some new features, without going overboard to the point of basically turning it into L2. To be perfectly honest, I primarily created it with my own level packs in mind, but saw little reason not to make it available for everyone to use.
Lemmini: This was (IIRC) around before Lemmix, and simply aimed to be a cross-platform version of Lemmings that was consistent across all various systems; there was no intention of perfectly mimicing the original behaviour, just of roughly estimating it and providing a playable Lemmings game.
SuperLemmini: This was created as an upgrade of Lemmini, to bring it more in line (though not 100%) with Amiga mechanics, as well as adding some new features (although not to the same extent as NeoLemmix has).
Lix: This is an entirely different ballgame altogether; the purpose here was (as far as I know) to create an online multiplayer Lemmings-like game; originally it was actually using Lemmings but was changed to an original character for copyright reasons.
So, as you can see, each of these projects essentially fills a different niche. I would, in all honesty, somewhat argue that Lemmini has outlived its usefulness due to SuperLemmini (whereas NeoLemmix doesn't so much replace traditional Lemmix in the same way, due to traditional Lemmix's aim of being an exact port), but given that the amount of content available for Lemmini is probably second only to the amount for traditional Lemmix, I doubt it'll disappear anytime soon. (There again, Cheapo had a lot of content, yet it eventually died out - though with that being said, there's currently ongoing work to port existing Cheapo content into the NeoLemmix engine.)