Its very rare that a carb wears out to the point where you have to replace the whole thing. It is sometimes necessary to clean it, or rebuild it with new gaskets, needle seats, etc.
Only once have i seen a carb so bad that it had to be replaced, and that was one my dads mid 70s JD tractor, the butterfly shaft had worn into the carb body to the point where it was just too sloppy to work well, and it was sucking in unfiltered air.