It's quite a stretch to simply allow free access to your city from a potentially malicious neighbor.
Yeah, and that's why they wouldn't. I don't see how having faction aligned transportation services not offering transport to enemy (or even rival) controlled zones leads to the idea that
any transportation service is unrealistic.
A number of those boats looked like little more than paddleboats. It's ridiculous to argue which system had more believable exploitation of the system than the other.
No, it really isn't. On the one hand, you are so weighed down that you cannot move so you cast a spell that lets you move for one second. Somehow this gives you the ability to travel for hours/days while carrying enormous amounts of loot on your person. On the other hand, you unload your good onto some type of vehicle, the majority of which are large boats (really the only small boats were the Vivec gondolas) or giant bugs and with their assitance are able to transport large amount. To argue that both systems are equally believable is absurd and very disingenuous.
Now, if you want to have a discussion on whether or not it's realistic to carry as much as you could in Morrowind
or Oblivion, we can talk about that. I'm totally for making adjustments in terms of how large quantities of loot is handled.