FWIW just heard a vet on a podcast that felt the clinical trials were not conclusive and did not vaccinate his own hunting dogs. I know that's a controversial statement, but this is a forum for discussion. With that said, I have vaccinated most of mine.
I've been using the vaccine on my dogs for the last 11 years so I would think there is plenty of data available. I've never seen any side effects except a lump at the vaccination site that goes away in a week or two and doesn't always occur from year to year. I've had one vaccinated dog bitten (by a rattlesnake) that went to the vet and was prescribed only antibiotics for the puncture wounds, no additional treatment was necessary. His swelling went down within a week and he hunted the next weekend.
I also snake train my dogs in addition to the vaccine.
As has been stated... cheap insurance.
edit note: the full effectiveness of the vaccine takes about 30 days to get going and if this is your pups first, they will give him 2 shots, 30 days apart and then a yearly booster after that. The vaccine only runs for about 6 months so you will want to make sure you plan for that if you bird hunt in the fall through spring, etc.