If refining gear is more expensive than mining gear, it doesn't merely make sense to concentrate the former - it makes sense to move it to a secure location where Bajoran terrorists can't get to it. A surface location would offer logistical advantages in that one would only have to ship the end product to orbit, not dull ore. However, you'd still also
need an orbiting location for your intimidation base and starship harbor. One integrated location might be better than two.
And the cost of surface-to-orbit logistics might be negligible if you can use transporters on the ore...
We still have a poor picture of how Terok Nor was supposed to operate. The DS9 Tech Manual offers some ideas, but few of these get onscreen support. How did the ore get up there? How did it flow through the station? What was done to it, exactly? Who did it - repressed Bajoran slaves (never mentioned in any context), or privileged Bajoran paid workers who were free to move (since e.g. Kira moved quite freely in "Necessary Evil")? How did the end products get to Cardassia - or was ore perhaps shipped as well? Was the station able to process multiple types of ore, or did it specialize, and if so, in what? What other roles did the station have?
Also, what other assets might the Cardassians have had in the system? We only heard of prison camps, one of which had some mining going on; we didn't learn how most of the mines were run. Surface garrisons, government strongholds and the like were never mentioned. Yet apparently the Resistance still occasionally had a Cardassian target to strike at, rather than merely lashing out on fellow Bajorans.