Why are some people adamant about not putting ketchup on a hot dog? This video suggests it's about the other condiments. I'd suggest it's about the sausage.
When I was a kid I put either mustard or ketchup or both on my hot dogs, although even as a youngster I was told it was wise to lay off the ketchup. I don't know exactly when, either when I was in high school or college, I did a little experiment. I had two hot dogs in one meal, one with just French's mustard and nothing else, and one with Heinz ketchup and nothing else. I was very surprised that the mustard one was easily superior. Hot dog sausages aren't exactly gourmet food, but they do have some subtle flavor. Your average hot dogs aren't a lot different in flavor from bologna, and I don't recommend putting ketchup on your bologna sandwich. If you want to taste the sausage, avoid the ketchup. If you'd rather not taste the sausage, then have at it. Better hot dogs like Nathan's and Hebrew National do taste better than your common Oscar Meyer, and I think it's a bit of a shame to hide that with ketchup. So, yes, I recommend saving your ketchup for meat with stronger flavor like hamburger, ham and left-over roast beef. (And for heaven's sake don't put ketchup on a good steak!)
My father had a job at a butcher shop in high school. He always said if I knew what went into sausage I wouldn't eat it. But he ate sausage and so I did too.