I don't give it and never have. It doesn't actually provide immunity to the pharynx tissues, so your horses still has a 50% chance of contracting it. If you do choose to vaccinate against it, you have to booster every 6-12 months.
One of my horses travels a good bit for shows, but practically never spends the night at other barns, I don't let him get close enough to other horses to sniff them. One will be traveling occassionally this year, the other is staying put. So, I don't even worry about it.
The Strangles vaccine is given based upon the risk associated with each situation.
This vaccine does not necessarily prevent disease but does lessen the severity of disease should a horse contract Strangles. This is because of the way that the bacteria attacks the body.
The Strangles vaccine also comes with increased risk of adverse reaction as compared to the vaccines for things like EEE, WEE or Tetanus. Horses who have had Strangles in the last year (even such a mild case that it wasn't noticed) can have a severe adverse reaction to the vaccine. If giving the intranasal vaccine, there is also the risk of having the bacteria get on the skin and contaminate injection sites if other vaccines are given at the same time.
You should always discuss the pros and cons of this vaccine with your veterinarian prior to giving it. Adult horses are at much lower risk of contracting Strangles than young horses. Horses that kept in facilities where lots of horses pass through such as breeding facilities, show barns, sale barns, etc are at increased risk of infection. Horses who have had Strangles in the past 5 years are much less likely to contract the disease again. So the factors that contribute to the risk of infection all have to be weighed against the chance of infection.
My horses get strangles and tetanus vaccines yearly. I have never had a problem with it, and from what I've heard if the vaccine is given properly and if the horse has never had strangles, it does lessen the chance of them getting it.
Its a personal choice, and is completely up to you if you do it or not. I agree with Ryle. Discuss it with your vet, because they can give you the best advice regarding your horse.