The issue with that is that is then carts getting sent on will eventually end up somewhere else beyond the load limit. If traffic is typically between two destinations, you could end up with a pile up of unloaded carts just beyond that station. When it is finally loaded, it would overwhelm the cart sorting at whichever station they appear at. In case you're wondering, 2-3 carts in very close succession could break sorting systems simply due to the length of a pressure plate reset.
Putting nodes more than the chunk load diameter apart would also introduce new problems, where the tradeback cart never even makes it back to the original station. More to the point though, the cost in a tradeback system is more in the sorting, storage and shunting system at each tradeback node then the wiring between nodes, (the reason I said long distance stations cost more is that they required multiple nodes).
Finally, we still have the issue with redstone bugs, they would be able to play absolute havoc with this system.
Look, since building each station isn't too costly under the current plan, I'll build them so that they are spaced such that we have the option of converting them later. We don't have to update, but if we get the resources to do so, we will be able to. Just so long as people understand that tunnels and station points are not to be dug near, unless you don't mind having them possibly filled in later.
Also, with the slums, can people please avoid building up onto the bluff to north? No one has yet, but some of the structures are getting awfully close (especially that tower... which also raises the question of why there is a narrow sculpted tower in among a collection of hovels anyway?). I say this because I will eventually be doing some work on the top of that bluff (extending city walls along the cliff edge, for starters).
EDIT: Huuuhhh. Doing some testing, it looks like the figure I have for chunk load is bollocks, on Far I can see almost twice what I should be able to. Considering the source I had said it was independant of render distance, that has me thinking.