I went to the party for Therese Raquin, and I don't see a reason for not going if you have tickets for that day anyway. It's a nice way to mingle with follow young-ish theater fans. And yes, there was an open bar (you had to show ID at some point I believe) and there was plenty of finger food.
Not sure where this party takes place, it was at some steak house when I went and it was a bit crowed in that space, but it was fun and my friend and I ended up chatting away until midnight or so, when they shut it down.