Brannon Braga explained the policy of no on set changes to the dialogue on an episode of Shuttlepod Show. The policy came from Roddenberry as a means of maintaining the timelessness of the script, and Berman embraced it because it would mean that there would always be clarity and concision in the complicated stories laced with fantasy technologies.
(That said, I think Picardo had some leeway, but I could be wrong.)
(That said, I think Picardo had some leeway, but I could be wrong.)