Some comics, like the "blue collar" dudes, are constantly doing gigs which demand a larger quantity of jokes, hence the lack of quality in their acts. So instead of sounding like a broken record, they pay guys like Ritch Shydner to write some corny ass shit for em.
Especially during this interweb age.. If you plan on seeing a popular comic who's currently on tour & want to spoil the experience (or worse, heckle dude by yelling out his punchlines), just youtube his name & search by most recent uploads. Chances are, he's doing the same act in each city he hits up.