I don't believe in over vaccinations. Titers will tell you what you need to know. I have heard that some states are considering titers be considered on required vaccines not sure how true that is though.
Years ago they did find a connection to the lepto combo shot causing HOD in Great Danes. No not a scientific study by the gov but one person spent years compiling information and showed a direct link to it.
If you have ever seen a puppy live with and ultimately die from the severe pain of HOD you'd think twice about combos and 7 in 1 shots of anything.:(
I think it is laziness and perhaps the idea of it being less expensive on the part of the owner to have all the shots done at one time. Which the companies are well aware of which is why they come up with these combo's.
I have since switched to a toy breed and Dr.Doug is absolutely correct about being vigilant and informed when it comes to vaccinating and meds with toy breeds.
Almost lost my MinPin to a rabies shot. Albeit that was most likely from the carrier not the vaccine itself. But when they are that small you can loose them from a reaction in a heartbeat. Unlike the larger breeds that can take a punch better from shear body size.
Most medications have a effective dose in terms of body weight of the animal; smaller animals get less drug. This is not the case for vaccines and may very well be an issue for some small animals.
X2
The same vaccine given to my Danes is the exact same one given to my 4 lb Min Pin.