Right... you don't want to force everyone to make significant changes to their parsers. If you are writing a C or C++ parser, you know up-front that there is lots of funny business in the language syntax and can plan your parser accordingly. If you are writing parser for something a bit more sane, like Java or CSS, then you can choose a much simpler architecture for your parser.

Part of this means that if, say, your language is LL(1) or something like that, you will want to keep future versions of the language LL(1). This can put you in a bit of a tight spot sometimes, when you're making backwards-compatible changes.