Also note that while Mare was an Allomancer, she wasn't what one would call a "dangerous" Allomancer. She was a Tineye, which isn't one of the top tier martial powers. She couldn't have used atium, and even if she HAD somehow found [tin], she'd simply have been able to hear and see better. Which would have made her better at finding the atium.
The Allomancers to keep out of the Pits would have been Lurchers or Coinshots (who could have destroyed the crystals), and to a lesser extent Thugs (who could be difficult to control.) Mistborn, of course, needed to be kept far, far away, lest they get their hands on atium.
There's more going on here, of course. If I ever write the Kelsier short story that talks about him discovering the Eleventh Metal, I will get into why the Inquisitors weren't given Mare as they wanted. The Lord Ruler specifically chose to send her to the Pits rather than handing her over to the Inquisitors. (Note: She wouldn't have ended up on a hook. Inquisitors had other...uses for skaa Mistings they captured. See book three.)