The Lepto vaccine has a very high incidence of severe reactions to it.
Lepto itself is very rare, seasonal,
transmitted only through the urine of infected animals/people coming into contact with the mouth, eyes, or open wounds (either from drinking urine-tainted water or other indirect contact with it), and is easily treated with antibiotics if you catch it early enough.
Because it is a health risk to humans as well, you can actually find out if there are any outbreaks in your area and take precautions accordingly. If you know there is Lepto currently reported in your area, and
watch for any symptoms in your dog, you can treat it safely. If you suspect Lepto, ask your vet to do an
antibody Titer for it, and if it comes back positive, start the fluids+penicillin treatments.
Really, there is very little justification to use a vaccine that only protects against 4 out of the 100s of known Lepto strains, has a very high chance of making your dog seriously sick, and only lasts 6 months. Even if your dog is not allergic to it, every time you administer it you are taking a risk with it's other known side effects.
Be aware that it is often included in the basic Distemper-combo (DHLPP) shot as well.
To be honest, I really kind of resent having to do research on these issues myself, simply because so many vets are either dishonest, or lazy in continuing their education out of school. The vet industry is so unregulated that it's easy for quacks to make a living and very hard to shut them down with malpractice suits. You have to do your homework and be very picky about what vet you choose and be aware of basic info on pet medication (ingredients, side effects, etc) for your own pet's safety. I wish it wasn't this way.