People who are traveling to NYC don't want to pack the extra clothing, run back to the hotel, iron it, change, etc. I can see that being a concern, especially if they've spent the day at a ball game or walking around Central Park.
In the main dining room I don't think I would wear jeans, but I'm not sure what their official dress code is. Usually if jackets are required, there is a no jeans policy. I would call the restaurant and ask.
i was eating in the bar area last week and saw this young very attractive couple leaving the main dining room. the man was wearing jeans and a graphic t-shirt under his jacket. and he walked by the host stand he handed them back their jacket.