Not truly needed, but worth it.
However... Cost-benefit principle: an individual should only take an action if, and only if, the extra benefits are at least as great as the extra costs.
We have to amortize for the docks, cargo shuttles and space centers. We do that by including those costs in the price of the goods we sell. Plus, we need to include the price of the space boosters and the cost of the goods. While we have no problem transmitting these costs into the goods and still remain competitive (on a small variety of goods), there is one resource that shoots a hole into Space Trading: time. This all takes time and patience, more time and patience than the world markets, thus removing any competitive advantage there is to space trading and as such there is no incentive to use this, expect some very few cases with high willingness to buy (nuclear weapons).
There is one very luring incentive to use space transportation, but not necessarily space trade... Weapons that enter through the space center into the country's stocks do not damage military spending budget. But this doesn't imply you have to buy the weapons at Space Stations, you can merely transfer them from one country to another or from world to world.
If things stay as they are... It just won't do.
Incentive principle: if the benefits of an action increase or its costs decrease, more likely it is people choose that action.
The greatest benefit increase will come from allowing greater amounts of goods to be exchanged within the same time frame. It will all be faster. That can be achieved by the following actions:
* Divide the volume each good occupies and the storage space of a shuttle by x
* Reduce the cost of space docks and allow them to be merged (or not)
* Reduce the cost of space docks and introduce some method to expand their storage space
* Multiply by x the storage space of a dock
* Any combination of the above
I believe it's better for x=10, because we all love the decimal system (and x>1 to avoid funny business). The amount of goods a shuttle can currently transport should not be increased, because of the weapons that can be freely transported. More importantly, the action that is chosen should be the one that better suits the system.
Short version:
The (normative) cost-benefit principle is being violated, push for the incentive principle: allow greater storage space in docks.
Taken into account the price of 200GC for a dock and a daily rent rate of 0.4GC, it takes me 500 days to get my money's worth back. I believe a price of 150GCs per dock would be better, as it would take 375 days to amortize the dock, which is little more than a year.
Terminology: Docks are available for renting and not leasing. A lease gives you the option of buying the object at the end of the contract. Although I haven't tested it yet, I'm certain the dock won't be available for purchase at the end.