Also, whats stopping the Rulers from having the ability to turn on and off the portals like a lightswitch?
A flick of natural thinkamancy, and the city is completely secure from the MK until such a time as the Ruler wishes to allow a caster to enter the MK, or a hired caster to visit his capital.
Since there has never been a need for this in the past, and also because casters are allowed more independance than most other units, it makes sense that some sides leave their portals on 24/7.
This would also explain why we only see between 10-15 portals at a time - some sides who have no need for the MK leave them turned off. Faq, a side who has long benefitted from secrecy, could be one of these sides. (except for a quick open and shut to allow Vanna access)
Charlie, although a reputable employer in the MK, would almost certainly have his portal switched off, if possible. If there was such an easy way to gain access to Charlescom, and so many casters who wanted him out of the picture, I cant see why he wouldn't have been confronted with an army of linked casters yet.
MK neutrality or not, if your upkeep is in on the line, and Charlie is dominating the thinkagram market... *shrug*
Now, if this were true, it could well be that the idea of the MK being used as an exploit will occur to Jetstone once they figure out that GK have targeted the dungeons. They might then flick the portal off and leave Parson stranded, with a bunch of angry casters between him and the GK portal

Anyway, this is based on pure nothing, and raised only to cast a shadow on the assumption that you could use the portals as an unlimited exploit.
No, no. It hit him in the brain because it killed him. - Dante