When I graduated from veterinary school in 1970, we vaccinated every puppy for Leptospirosis. Over time, the incidence of Lepto decreased to the point that vaccination seemed unnecessary so most veterinarians deleted the vaccine from our core program.
About 6 years ago, the number of Lepto cases began increasing but the problem was that the newer cases were caused by a different "strain" of the organism than the old vaccine contained. Eventually, vaccine manufacturers developed new vaccines that used the more recent germs that were causing the disease. As a result, we have updated our vaccination protocol to include Lepto vaccine in some dogs.
Here is my reco: If you dog is a outdoor type, i.e. hunting or field trial dog, vaccination with the new Lepto vaccine is important. The newer strains are carried by deer, cattle, and horses, in addition to the rodents that have always carried the disease. In the past, if a dog was not exposed to rat/mouse urine, there was little reason to vaccinate for Lepto. But, with the new vaccines, dogs that hunt around deer and livestock are at risk.
If your veterinarian recommends Lepto vaccine, make sure the vaccine contains "all four serovars". Lepto vaccine is a bactrin which is an older technology. As a result, it does not give the long-term protection that vaccines for viral diseases do. We give two vaccinations about 30 days apart and at least one annual booster to insure protection. Leptospirosis is a serious disease that can damage many organs but especially the kidneys. I think the vaccine is worth it and routinely give it to my own dogs.