Actually, the L-numbers are a COMMERCIAL and concrete way to identify a member of the family Loricariidae. Several L-numbers identify morphs and color variants not scientifically described. It's a system created by german pleco-lovers/ sellers/ exporters (in Datz magazine, if I'm not wrong) to identify a specific pleco, even if it's not scientifically described yet. Anyways, it's very useful to fish keepers, scientists and sellers/ exporters/ importers.
with so many new and undescribed species being imported they had to come up with some way to keep them straight. so it was easier to create the L# system than wait for the scientists to have the time to study and describe each species. Like Daniel alluded to, some different L#'s are actually believed to be different color morphs of the same species. For example, L75 and L124 are believed by many to be the same species.