On a large enough station, so what?
Adding a refuelling system and 1ML tank to a 2.5MT station increases the size and cost by less than 0.1%.
So, sure, by the time you build 999 of these, you could have built one more if you hadn't been so wasteful.
OTOH, if just once it comes in handy to have that fuel available, it was probably worth it.
But by the second one you could have paid for the tanker, which is useful on its own.
Since we both seem happy to whittle away at this exceedingly fine point, I'll follow up on this.
You provide an interesting alternative.
A tanker does provide a lot more flexibility. It can go off and refuel itself if needed, or go refill an empty fleet, or perform an emergency lifepod rescue, among other things.
Let's suppose our station has 500 orbital mining modules (plus the
de rigueur bridge and engineering space).
In my Add-Fuel-To-Huge-Stations plan, I add a refueling system and a Fuel Storage - Very Large.
Station weight and cost: 2,541,848 tons; 67,353 BP.
In your Fuel-Tankers-Sold-Separately plan, nothing is added to the station.
Station weight and cost: 2,540,317 tons; 67,326.9 BP.
My extra cost is 26.1BP per station.
My station weighs .06% more. Effectively zero change in tug speed.
Let's say a cheap tanker has a single size-25 engine @30% power, plus a refueling system and a Fuel Storage - Very Large (same as I put on my station).
At Improved Nuclear Pulse tech, the engine costs 11.25 BP. The tanker moves 1492km/s and has a nominal range of 2,871.4Bkm.
It's not a great design for long-range fuel hauling (too much fuel burned vs hauled on long trips) or for military fleet logistics (too slow, too small). But it is cheap, and if our only intended use is to park it near our orbital fleets, it's fine.
Tanker weight and cost: 2,513 tons; 72.7 BP.
You wouldn't quite be able to afford one with the savings from two stations--you'd need ~2.8. But for what I assume was top-of-the-head, back-of-the-napkin guesstimating, you were close enough to be called correct.
There is the cost of the shipyard to consider.
2400 BP is quite a hole to crawl out of.
If you are making this shipyard just to avoid putting fuel tanks on big stations, I'd say it's never going to be worth it.
But the shipyard employs a quarter-million workers. If you have more workers than you have others uses for, this is a boon. You make worker taxes from it.
(If you have a worker shortage, on the other hand, this is a problem. Also, please tell me how you manage to have a worker shortage. I can never create jobs fast enough in my empires.)
At the end of the day, it probably just comes down to playstyle. I like the simplicity of having an emergency fuel tank on my orbital stations. I have enough other tankers moving around my systems that the extra flexibility of parking a cheap tanker with my orbital fleet isn't going to add much value.
