Everywhere I externed I got an interview at and at all of the places at least a couple attendings/residents remembered me. I heard some places only interview externs, and other places show preference for those that externed. I would extern for the following reasons: 1. Learn more about OMFS in general/ show commitment to the specialty 2.Show that I am interested in a particular region or willing to leave my home region 3.Show that I am interested in a particular program 4. IMO the most important, to learn if the programs I am interested in are truly a good fit for me- you can get a good idea of the pros/cons of a program when you spend a week there. Every program says they have broad scope, good resident culture, good autonomy etc. but seeing things in person definitely made me re-evaluate.
I think there was a journal article that said roughly 50% of matched applicants go somewhere they externed at- for better or for worse many people end up choosing the devil they know rather than risk gambling it on what is promised on the interview day. This applies to programs too. Nonetheless, no program is perfect, and there are many great programs out there, externing is just a way to get more information in a less formal manner.