During the colonial days, when the spice trade was rife and the East India Companies were supreme, many of the ideas you proposed were instated. Traders did in fact have to pay rent, docking fees, handling fees, administration fees, etc. etc. etc.
The traders were also required to learn the trading languages of the world (resulting in extremely multilingual individuals) as well as carry hugely different forms of currency for different trading areas with no real exchange rate to go by (we're talking shifting between animal dung in one place and coconut shells in another).
Most of the time, ship based traders also had to wait several months after trading because the seasonal winds weren't blowing the right way. Thus, lots of interracial marriages happened ("I've got a wife in Cambodia, one in Kowloon, one in Havana, and three in Saigon!"), lots of migration, and lots of weird looking, rainbow-coloured children were spawned.
In many cases, traders also acted as spies to whichever lord they served. By easily mingling with the society of the potentially-invaded, they would have an advantage when the time to strike came. Even better, sometimes the influence of traders would make it possible to actually just assimilate the entire enemy into your empire and the natives would still think they'd be in charge. *hah*.
Basically, what I'm trying to say is that I agree with your idea that unless traders contribute something... ANYTHING else to the game other than look pretty after delivering a minuscule amount of your severely needed chalk, they should just butt off and bother someone else.