Actors That Manage to Play Themselves in Fiction

Charlie Sheen and Tracy Morgan play characters in sitcoms named Charlie and Tracy respectively, but not Charlie Sheen or Tracy Morgan.  However, the characters they play in these shows seem to perfectly mimic the characters they are in real life.  Charlie is a sex-crazed alcoholic.  No one can really figure out whats in Tracy's head.

Is this the height of laziness?  Can the writers do no better than to mimic the actors' lives?  Can Tracy not understand his lines if they aren't written under his own name?  Have you given up as an actor if you rip your own tabloid headlines for show content?  I actually enjoy both of these shows, but it might be the end of the line for both of these guys.  How do you go on to play a different character when you are known so well for just playing yourself?