I appreciate the feedback guys, both supportive and contradictory. The suggestion to reduce the magnitude of a slow (would not be able to affect snares) is a valid idea, however if this was combined with a tenacity item (Eleisa's Miracle, for example) it would make you nearly unstoppable. This would also apply exclusively to champions, so it would be valuable both in-lane and in team fights.
The fact that merc treads not only give you tenacity, but also magic resist makes them far more overpowered than nearly every other boot. Boots of swiftness give you speed, and only speed. To be competitive they just need something more.