Well, also, an industrial world with an E-port might use said port for air and ground transport too. The rail lines and air freight lines have hubs, why not fit in the extra room for that one ship a day with its tons of valuable cargo, have it on its way to its destinations pronto?
Ah, and perhaps that ship-a-day is carrying 100t of raw goods. The factory should probably be very close by, and next to the rail which moves raw and finished goods from the port to the market(s).
And although an E-class port isn't fancy, it might get significant traffic if there were other factors to outweigh its primitiveness.
IMTU they're not easy to find, though. I can't find any in the Spinward Marches.
Theoretically, a world could have an E port, a high population, a good TL, industry, and lie on the X-Boat route, for a traffic index of 3. If it were near a world with an A port, high TL, rich, agricultural, and also on the X-boat route, its traffic index would be 5, and the traffic through the E-port would be around 1000 people and 10,000 tons of freight per day. Since that's a downport, all of the traffic will be in small ships -- 600t, 400t, and 200t -- which probably means around 100 ships per day.
