As I thought a little about this yesterday, two things settled in my mind:
1. Sigil is definitely the Casablanca of the Multiverse. There are several establishments much like Rick's (yet even more so), and these establishments would be hard pressed to do business in their preferred manner under the authority of the Efreeti (too militant) or Erathis (too civilized).
2. The food supply situation in the planar cities really bothers me.
I realize I give this more thought than most, but when doing world design I think about food, economics, logistics, water supplies, etc. That's just how I'm wired.
And Sigil has no good way of reliably bringing in food. Or growing food locally. With the gateways changing all the time they can't get to a regular market, or get to markets during the right seasons. Why would farmers bring produce to a time and place when there's only a 10% chance of a buyer being there? It really isn't feasible to supply Sigil with food. Non-perishable supplies like building stone, tools and weapons can handle being cut off for a few weeks or months, but what kind of food stores does the city have? 250,000 people eat a lot of food.
Waste is also a problem. 250,000 beings (some of them size Large) eating all that food produce a lot of poop. Where does it go? Maybe there are mulch caverns beneath the city? I can't (or refuse to) imagine the smell.
Water is much less of a problem because if you imagine a couple super-big cisterns buried beneath the city acting as collectors for all the drains and gutters you could have a reasonably sustainable hyrdo-cycle once the water was gated in from some freshwater lake on another plane. You'd need some way to make it potable, but that's manageable. There's probably some helpful gelatinous beasties floating around in the cisterns that eat up all the waste and excrete Evian-quality H20.
Also, Sigil is really sparsely populated. Given the approx diameter and tube-size given on MotP pg. 27, my calculations show that Sigil has an internal surface area of 145.083185 square miles, or 375.763724 km^2. With a population of 250,000, that's 665 people / km^2. By comparison, New York City has 10,452 / km^2 and Mumbai has 22,658 / km^2. (
Link) Hardly the teaming metropolis. It sounds more like one of those suburban developments with a 1/8 mile strip of land between Big Box Stores.