The title pretty much has it down. I tried to summon a zombie with slowness 7, but nothing will work, not even splashing it with a splash potion. It shows the particle effect, but ignores it completely. I'm pretty sure this is a Bug. @Mojang Please Fix!
Is this still a concern in the latest Minecraft version 13w48b? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.