L-numbers were invented by German hobbyists/importers/magazines to try to get a consistent naming convention going for Loricariid catfish. The problem is that there are a heck of a lot of these fish already around, and still morebeing discovered and scientific naming is still a long way from catching up. Plus often importers/catchers are the first people to see these fish and they need a way of describing what they have, and making sense of what they order for.
Common names like zebra plec don't make sense here, there are jsut too many - it's ok with zebra plec,but clown plec describes a few different species, gold nuggets are 3 + different species and Bristlenoses are a whole load of things. You soon reach the ridiculous - Red Seam Gold Cactus pleco....
You can't do what they do for apistos either, which are ina similar state ie Apisto 'Rio Tefe' or Apisto. 'Blaupunkt' as you're not dealing with a single genus.
There are problems with L-'s, ie same species, different numbers, same number=different species depending on location, truly defining L-s, but they work relatively well, are reasonably well documented and for me they make sense.