A few years ago, Edward Albee retooled his classic play Who's Afraid of Virginia Woolf. It earned him a Broadway revival, a national tour, and some criticism. People felt that the changes were unnecessary and diminished some of the punch of the play. In essence, once Albee had published the original play he gave us ownership of his story and by changing it on us, we felt that he was taking something away from that.
I'm very opposed to playwrights who continue to tinker with their work. I know that there are always little things one feels that he or she didn't get quite right, but it leads to artistic stagnation to not be able to let go. If you are continually obsessed with getting your work right, it takes energy away from your future work. You become less productive and your relevance will begin to wain.
I also don't like authors who are over-protective of their work. Albee is a perfect example. He has to approve the casting of all major productions of his work. The late Samuel Beckett and his estate guard his works with an iron fist allowing no interpretation that they don't like. While I am not suggesting that directors and actors be given full licence to disrespect the playwright, but theatre is an art of collaboration and growth. There should be some room for exploration without the playwright policing every possible nuance of his or her scripts.
However, what Albee did in he rewrite of Who's Afraid of Virginia Woolf, was nothing short of genius. The play has become somewhat overshadowed by the film version that people are coming into the stage play with certain expectations. Albee has been able to slightly change the nuances of the play to make it stand alone again and reinvest in the relevance of the stage production. Albee was able to breath new life into the play and create a dialogue between the two mediums.
So there are some of the pros and cons for a playwright's continued involvement in their work. What else can hamper or embellish the life of a script?