L numbers are given because they haven't been asigned a name by science... so we hobbiests have L numbers. There are hundreds of unnamed species, it's just a common name.
L numbers are just an easier way to double check that everyone is talking about the same pleco. They aren't always accurate, and sometimes just wrong as changes have occured. The L stands for Loricariidae, which all plecos are a part of.
yeah PC is great, ive been able to identify my plecos. the only 1 the lfs's got right was the 128 blue phantom. but you do have quite a few that look simular.