Yep, agreed. It's not so much a question of whether an agency would want to accept a slowed down version but more a question of whether or not a potential BUYER would want it. Certainly, if the original clip was shot at a high frame rate, 60fps or even more, any potential client would take that into consideration for slow-mo potential.
That said, if the clip is really only viable by virtue of it being slow motion, then that should be the only clip uploaded.