Actually ACL did previews when it was running off-Broadway. If you read some of the history, you see that bits of the show were changed due to audience reaction.
For example, Dance Ten, Looks Three was originally called Tits and Ass. However, the creators realized that they would get a better reaction from the song if the audience didn't learn the punchline by reading it in the program.
Some people credit Marsha Mason as being the one who convinced Michael Bennett that Cassie had to be one of the performers selected at the end. Early on it was said that Cassie was not one of the final chosen.
And Neil Simon was brought in to write a few funny lines to punch up the script. Pam Blair who played Val mentioned one time that she thought what Simon had written for her was awful and persuaded Bennett not to use Simon's line for her character.
If anyone ever tells you that you put too much Parmesan cheese on your pasta, stop talking to them. You don't need that kind of negativity in your life.