There is no motivation for okie to keep him unless they plan to either resign him or sign and trade him, which is unlikely since joe wouldn't have that higher trade value and secondly it's doubtful any other team would be motivated to try and trade for smith, since they could probably offer him more than enough money using the MLE in the off season.
Secondly there r teams out there that would offer joe above the vets min, so he would be motivated to give up some salary, not to mention the added bonus for him to be able to showcase his skills during primetime/playoffs, just before him hitting free agency.
i doubt 33 year joe smith is in the long term plans of the thunder, therefore a buyout would make too much sense for it not to happen.