I'm a dentist in the Navy. Last fall I saw some numbers regarding the retention rate. I can't remember the numbers exactly, but I believe it was something in the 16-18% range, so I'm not sure there's much improvement there. In the Navy it seems like everyone who stays out of trouble gets to 05, while 06 requires "playing the game" I think lately around half of those eligible have been getting the 06 promotion.
If you don't "specialize" in the Navy, it is pretty unrewarding professionally and financially. I put "specializing" in quotes because there a lot of programs that are not your traditional residencies. These include operative, radiology, pathology, exodontia, comprehensive dentistry, etc. Obviously these programs don't translate well to private practice, so these types tend to stay in.
What happens is that if you take the plunge into one of these programs, it ties you up with Uncle Sam for many more years. So it's kind of an all or nothing decision.
I don't know many LT's with plans to stay in. If you want to be a general dentist and focus on clinical dentistry and develop treatment plans seeing them through, it is just too limiting. Also the moving around every 2-3 years bit is a big deal breaker for most.