At this point, I'm not sure if Sulu and Uhura were ever intended to have a bigger role than what we saw eventually on screen. For example, Fontana removed a big part for Sulu in
This Side of Paradise long before writing that second version of the Guide. In the original outline, the romance subplot belonged to Sulu, but she gave it to Spock in her rewrite.
As crazy as it may seem, a fan counted recently the lines of each character in the three seasons (
) and I don't see a significant decrease in their parts. Uhura had, in fact, more lines in season two than one. While Sulu's decrease during that season was due to Takei being absent from production for a long while.
In view of this, the claims that Shatner started stealing lines from other characters as the show progressed, doesn't seem to hold much water either. Scotty's role, in fact, increased dramatically.