MS is a great stat but it is situational and works far better for some champions or roles.
If your a heavy gank champion who needs to be able to move across the map quickly then obviously having high MS allows you far greater map control and ganking power.
If your someone like Poppy or Singed where your designed to be quick then the extra MS is great. A 600 MS poppy becomes a monster who can easily place charges at will and hunt down squishies easily.
As for being slowed or whatever, if your a champion that relies on MS such as poppy, singed, rammus and so on, cleanse is a very good idea to use as a summoner skill.
MS is situational though, does Morganna, Kayle, gangplank and so on need to stack MS...obviously not.
All depends on your champions and the role your playing that determines how mobile you need to be.
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