I have to be the voice of reason here. Firstly, choosing a medical specialty, in most cases, is not as definite a thing as applying for medical school. For most people I know, they would be relatively happy with more than one specialty. This means that after trying to get a certain residency in a specialty for 2 years in a row you still can not find one, going into another one is not as different as not going to medical school.
Of course, if you are dead set on a specialty and cannot imagine doing anything else, then by all means try longer. There is no guarantee that you will find a spot and may have to face residency in another specialty 4 years later (this would be at age 29 for me and older for the average med student).
My second point is much more practical. Many medical students these days are in debt over $100,000. As I mentioned above, most med students will be 25-35 when graduating. By doing research or another residency while attempting to find a spot in the "only" specialty for you, you are delaying the payment of these loans. Also, deferment is only during residency and is allowed for a limited time (5 years I think). So you could face the reality of being a 30-40 year old with a 40K income and tons of loans, with interest hanging over your head and a good income still years away. Not the situation most people want to be in.
I say try for a couple years, but if it doesn't happen, there must be other specialties you will be just as happy to practice.