I think a lot of the reason for the slow was to go with you're ult. If you didn't have the slow you could just keep up with enemy massivly slowing them for already more than what you are slowed for.
Think about it. You'll roll up turn your ult on than taunt defense curl so you'll prolly get 3 hits of your tremors on them before they can start go get away so 3x8 at level 6, to give them a 24% slow, by the time they get out of your ult it'll prolly hit them twice more so 40%. You can unactivate your defense curl to drop the 60% slow thus allowing you to get all 8 hits of your ult for 64% for a extra 1.5 seconds after your ult ends. Generally at level 6 your ganking a lane so you should have your laner with you getting free damage. If they decide to not run after the taunt you have no reason to turn your defense curl off either.
So like I get why the slow is there. Keep in mind at level 11 that ult slow turns into a 80% slow and at 16 its a 96% slow.
I do think it should be lowered though.