Just my 2 cp...
Speaking strictly from a RP perspective, I think the ports should work regardless, and here is why.
The port items are a reward to those who have proved thier alliance to a particular cause/faction. A high ranking official in each of several cities gives out a key to the city gates to those who are considered trustworthy. Sometimes this official errs and gives access to a spy, a traitor, or someone who just is not trustworthy. When this happens they have given an enemy easy access to their home, and they can't just change the locks because all their ally's have the same key as well.
Basically they trusted the wrong person and end up suffering the consequences for it.
As far as my personal RP goes...
I could get the grobb port item without hurting my newport faction (in theory) if I had someone who was already kos to newport kill the mob for me that gives bad faction. A quick illusion and I would have access to grobb, and with a bit of luck, dubious faction as well. As tempting as this is, since a shortcut to Mielech would be convenient, I can't quite justify it since my character would never kill an ally of newport. On the other hand, a particularly dishonorable spy might secretly kill an ally in order to gain the trust of the enemy, then turn on the enemy after they have the access they need.
I cna justify swapping between giant and dragon factions, as they are both rather violent races which my character dispizes. I don't mind slaughtering one this week and another next week depending on which one suits my needs at the time. From their own perspectives, even if they don't trust me due to past crimes, they would still be willing to work with me and arm me for battle when ever I appear to oppose their enemies.
After all, governments in the real world frequently ally themselves with (and sell weapons to) their enemies simply because it suits their purpose at the time.
