I'm not sure where the balance is. Too many is certainly bad for a city as well. LA has seen a big increase in strip club billboards. Not sure why that popped in my head.

I actually enjoy the ads on trains in Tokyo and learn about all kinds of stuff from them from products to events at museums. I noticed the lack of ads in the Paris subway (in the cars themselves) left me kind of bored. It doesn't help there's no mobile signals in the subways.