I am certainly not the expert since I just interviewed last week, however, here's what I've gathered from the forums and other sources:

For certain folks it's at the discretion of the interviewers. Things that affect the decision include the interviewee's background, recency/type of flying, and how you perform on other parts of the interview process. Meaning, that even if you have a solid background, but stink up the written test/technical interview/CRM exercise/etc. they could elect to have you do the simulator. Conversely, you might not have the strongest background, but if you ace every other part of the interview process they might elect to have you not do the simulator.

Again, I'm not the expert, but that's what I've gathered. In my opinion, you would be foolish and unprepared if you show up assuming that you WON'T do the simulator.