A lot of it has to do with having more than one parser for the same thing -- it's bad if they don't agree on the meaning of the input.
That has everything to do with grammar theory (and with how clumsy parser generators are and with how brittle many hand-written parsers are and how often they don't quite implement the spec).
That has everything to do with grammar theory (and with how clumsy parser generators are and with how brittle many hand-written parsers are and how often they don't quite implement the spec).