his statement holds some credit, because as was stated, the L number was simply for the use of identified plecs that had not been given a scientific name. (i mean L and LDA numbers just came from german magazines as they had nothing else to write it as). some spp. was designated a name almost straight away, and others not. Luteus does not have an L number for this reason. Since the L numbers etc, there has been many plecos described and designated a specific scientific name (and some unspecific only referring to the genre of pleco). However datz also gave some L numbers to plecs already described, as to make a list.
the list also makes it easier to refer to each specific pleco, without using unspecific common names, or learning all of the scientific names of such an extended spp. Obviously as it was divised from a magazine there were flaws. Some being they have named the same pleco twice, such as the zebra pleco, and is thought that they are simply one off mutations. On the other hand, there is the gold nugget trio, all having different L numbers, and rightly so, as they all come from different places, and display slight differences on patterning. Then there are, as wyldfya said, the 2 L200s, which have since been found different sp. but as they were both given scientific names, there was no need to designate them into 2 different L numbers.
The L numbers were chosen to be given to different plecs that showed different colouring, markings, shape and size to any other. obv mistakes get made regarding above. The L numbers have no specific scientific credit, as no tests are carried out to identify them as species. if a plec is given the scientific name, l numbers definately become obsolete, but still hang around just to use as short hand