As long as you know ahead of time, I don't see it as a problem (like the permanent ending of the main quest).
I still don't like it. One of the biggest problems I had with the Pitt was that once the trogs were dead they didn't respawn. This is even worse, once you leave, you can't go back. And it's not even a simulation like O:A. I'd like to be able to go back and take some time to explore the casino and fight ghost people and such.
I'll still get it, since I can replay it with multiple characters and the new items are cool. I'm also looking forward to trying to find some sort of exploit to get all the stuff out of the level.
I guess we'll just have to consider it the "O:A" of new vegas and hope that future DLCs have towns and areas to explore.