In general, I feel that they help more than they hurt. When they hurt, they hurt profoundly, as Roxanne who writes Champion of My Heart has found out when her dog, Lilly, had an extreme negative reaction to her routine rabies vaccination (500 days ago, as of June 4). Lilly has been in medical distress since then, has almost died, and has had one severe relapse. Roxanne writes that Lilly can never have another vaccine in her life.
(picture from Wikimedia commons. You'll see "why a tiger?" after the jump)
When people don't vaccinate, though, that's a problem too. Rabies vaccinations can sometimes be waived, if your state is one that allows titer testing in lieu of vaccinations. Titering confirms that sufficient active antibodies are active in your dog (or cat) to preclude the need for a vaccine booster. If your dog has neither an up to date rabies vaccine or a titer test, and bites somebody, you and your dog could be in big trouble.
Other than rabies, though, not vaccinating for canine distemper can cause unforeseen difficulties. Canine distemper is communicable to big cats, for example. The first tiger to have it was in 2003 in Russia, when a tiger "walked into town and sat down". She appeared healthy, but was profoundly brain damaged, and died in veterinary care six weeks later. It was previously discovered in a wildlife preserve in California in 1992, and before that in 1988 in Iowa. This latest round of tigers with distemper is in Sumatra, which thankfully is an isolated island population. There are few tigers in the wild, and fCanine distemper is also considered responsible for the deaths of 1/3 the lion population in a Serengeti preserve in 1994; it is believed the disease was communicated to the lions from the herding dogs kept by the Maasai there.
So, in Sumatra, Wildlife Vets International is starting a study both to monitor the diseases carried by tigers, and also start a vaccination campaign. Evidently, "In the research area, only 16% of dogs were vaccinated against the virus despite the ready availability of the vaccine. Of the non vaccinated dogs, 58% were shown to carry the virus." Hopefully, WVI will be able to influence people in Sumatra to vaccinate their dogs against canine distemper. Dogs can be non affected carriers (obviously), or they can contract the disease and die from it.
Interestingly, distemper is related to measles, which some humans have stopped vaccinating their children against for varying reasons. As a result, measles has been making a comeback, and much like distemper, measles can be deadly for a child; the CDC states that for every 1000 children who get measles, "1 or 2 will die".