It will add a balance point. Halt will be balanced around the most mobile champions and the most mobile champions will be balanced around halt.
This is the exact case with healing and grievous wounds currently. Dr. Mundo is balanced around getting ignited when he is ulting. If he is, his ultimate still has to feel good. However, if he isn't and he ults, his ultimate is insane, you NEED to apply grievous wounds to kill an ulting Mundo.
The same thing could happen with halt, where both the halt and mobility are overtuned, so if you get halted, you feel bad, but if you don't, you get to run free every time. If you don't overtune halt, it's just a very bad summoner spell and ignite will be superior.
A last point, these champions could just take cleanse and completely negate halt (doesn't work for Dr. Mundo because cleanse does not cleanse the grievous wounds part from ignite, only the damage).