Workers coming up with their own stuff used to happen. The process of the writers scripting out things word for word has now taken over. Some workers, Jericho and Michaels, for instance, have far more leeway in scripting their promos than others.
The major/critical problem about totally scripting a promo or even a match (This was discussed when the TNA script was leaked and the matches were written out, move for move) is that it doesn't allow for the talent to improvise if the crowd doesn't react positively. If the audience starts to crap on a segment and the worker can't improv his way out of it, he'll either panic or continue to give a promo/segment that is already bombing.
That's why no one under 40 can cut a money promo.