I have only been asked for Resort ID on 2 occasions when boarding Disney Transportation. (This is in over 40 trips over 12 years.)
Both times have been at closing time of the Magic Kingdom.
The first time I was heading to the Grand Floridian for dinner and wanted to board the resort monorail. They asked for my resort ID.
The 2nd time I was heading to the Polynesian for dinner and wanted to board the boat that goes to GF and Poly. I WAS NOT a resort guest (short offsite trip), but told them I had dinner reservations at the Poly and was allowed to board.
When large numbers of people are trying to leave the Magic Kingdom, they sometimes restrict "resort transportation" to resort guests.
One other time I was entering the TTC early one morning. There was actually someone at the gate asking to see park tickets or resort IDs. I'm guessing if someone didn't have either of those, they would direct them back to the ticket booths there at the TTC. However, if someone insisted that they were just going to one of the resorts to dine, I'm sure they would have been allowed through. I think it was more of a courtesy to make sure guests had the tickets they needed before proceeding.