Immoral?? Try illegal!!
I recently was in the same position. I normally do not vaccinate for strangles but a barn where my coach was giving a clinic required it, so of course I did plenty of research.
My vet and all the stuff I read recommended the nasal vaccination. The vaccination given first time will not be effective unless you give two doses, 4 weeks apart and after that it takes about 2 weeks for full effectiveness.
I was also initially very worried about giving it, but after talking to my vet and reading up on it I decided to do it. My horse just got his second dose and is fine. As long as your vet is experienced in giving the vaccination and you are diligent in being at the barn every day to monitor him, it is unlikely that anything will happen. If you do notice any listlessness or other behanvior changes, take his temperature immediately. If it is elevated call the vet immediately. When strangles is caught very early it is rarely fatal.
About it spreading to other horses? The dose of live strangles is so low that your horse won't catch it when it is put directly into their nose. Another horse will touch it (or even your horse's nose) and be completely fine. There is a tiny bit of strangles in most horses noses. My horse has been turned out with another unvaccinated horse for the entire course of inoculation and they are both fine.