I can attest personally to the effect of seizure medicine. When I was 42, I started having seizures out of the blue. No family history, nothing showed up on the MRI, EEG. My neurologist put me on Tegretol (after trying a new med that I was allergic to).
It was awful getting used to it. At first I felt extremely drugged: heavy in the limbs, dopey tired all the time. The worst part though was that I would lose my train of thought in the middle of a sentence and couldn't remember what I was talking about. I also couldn't think of words when I need to. (Worse than the old age thing I have now). I adapted eventually, but the memory/word loss never really went away.
After four or five years my doctor took me off. Because of the type of seizure I had, my brain reset after that many years on the medication.
If I didn't know how invasive the medicine was, I would have got it when they took me off. They took me off by half a tab every two weeks. Each time I went down that half tab, I would get sicker than a dog (no pun intended!) for about a week. It was like having the flu.
I am thankful I was on them, because I have been seizure free for 7 years and the doc says I shouldn't ever have another one in my life.
I also work with students who have seizures and know how important the meds are.
The long and short of what I'm saying is, if the vet is saying they don't put dogs on meds unless it's serious, this is probably why. They are very, very strong and cause problems. It's the weighing of the options of benefit vs side effects.
I too think you should get a second opinion, as others have said. For your peace of mind and to assure that nothing life threatening or serious is wrong with your dog.