Is bribing off the robber a good addition to Settlers of Catan?
I have played this and it absolutely degrades the fun factor of the game.

You end up getting everyone paying off to not get robbed.  The person who will pay the least gets the robber.  We had the person rolling the seven getting huge amounts of resources.  

It really disrupted the flow of the game.

NOTE: we did not change the rules to allow this.  It was done as an agreement that was payed at a legal trading time.  One fun aspect is that, because it was not part of the rules, you could renege on your "payoff" commitment.  (Though this meant you were a sure target the next time around.)

While there was some fun elements (and gave us a few really fun games), I would absolutely not recommend it for more that variation in a game or two.