I hate Trading Post spam so much, which is odd because I never hated cottage spam, though now that I think about it I kind of disliked it. Cities shouldn't cover the entire workable area. Even in the modern area I prefer to have a one tile city. I think it bothers me because it makes no logical sense to have multiple trading posts within a city radius. It's just nonsensical.
I'll definitely be looking for a mod to be rid of Trading Post spam.
Being raised in the Detroit metro area there were at least 8 "trading centers" in the greater geographic area - they are more commonly known as MALLS...
The main improvement I'd probably make would be to tie the construction of trading post to city population - though it is quite probable that most balanced cities will naturally tend toward a reasonable ratio anyway. Just think of a "trade spam" city as a major commerence center for the entire civ where merchants from all over come to trade; and also where the major guilds have their houses.