I don't plan to write parser for real huge languages, just to be able to write small-but-complete parsers for any input language my code needs. An XPAth parser for exemple usually does not process large input, or a simplified interface that emits SQL queries, or a simple expression evaluator. For all of those needs the current regexps are not quite powerful enough, and writing the interpreter using Parse::RecDescent or YAPP is sometimes not justified. Especially as using native regexps will make it more acceptable for whomever has to maintain the code.
So for most of the needs I have, the speed of the parser is not a problem, I just like the fact that I will be able to write cleaner and more powerfull "very-little language" interpreters directly within my code.