Ruin didn't need them at all.
TenSoon kills OreSeur and replaces him at Zane's command because Zane owns his contract, and Zane is obsessed with Vin.
While Ruin is influencing Zane, and becomes obsessed with Vin because of it, he actually does so because Vin is the only person Ruin isn't pushing him to kill. Ruin wanted or needed Vin. Zane latches onto that absence anyway, simply because he thinks Vin is making him less crazy.
Ruin never needed a "spy" on Vin...
...because he already has a link to her. Vin's earring has been in place for a long time, and only right at the end when Ruin has nearly won does anyone realize that it's an active hemalurgic spike. She's been linked to Ruin the entire time, except brief moments when the earring falls out.
By implication, since Zane wasn't given any other directions... it seems Zane was just someone Ruin screwed with to pass the time. To House Venture, he was an unstable tool and weapon. To Ruin, he was a toy, maybe a test, nothing more.
In the long run, this does backfire indirectly on Ruin...
... since TenSoon becoming loyal to Vin instead of his contract was critical in the last book for both warning the kandra about Ruin's control, and putting Sazed and the kandra together.