I don't know. That's a interesting question. I just had the vet out today to vaccinate my 9 month old colt for the first time (well, he did have a tetanus shot last year when he was gelded).
I just asked the vet what he needed and the only things he offered were the 4-way (tetanus, influenza and Eastern and Western Encephalmyaltis(sp?) and West Nile. He never even mentioned Strangles.
I did ask a good friend of mine about vaccinating for Strangles because she has owned a lot of young horses, and this colt is my first, and she said she has never vaccinated for Strangles nor has he ever had a horse come down with Strangles either.
So I dunno. You kind of have to weigh the benefits with the risks. It would bother me a little to give a vaccine that has a chance of making my horse sick. That would worry me too. But then again, when West Nile came out, there were all these horror stories associated with that vaccine too. And I ended up giving them the West Nile vaccine because we were having cases in our area.
So I understand your concerns. I guess ultimately, if it were me, I would go with what my vet recommended. I have faith in my vet that he knows what are the disease threats are in my area, and what he has been treating other horses for, so he should have an idea of what my horse needs and doesn't. So, if I thought I had a good vet, I would go with his/her recommendation.