To be fair (and I play a lot of Sion), he needs a little bit of a love tap like they are giving him. He is REALLY strong if you practice with him, and most people don't realize just how much damage his E does, especially considering it shreds armor and makes it very hard to dodge his Q (which together means that his Q deals a lot more damage if you E first). As long as they don't touch its utility, it will be fine and Sion will not be weak.
Additionally, Sion jungle (where I play him most) is disgustingly strong right now (I'd say stronger than Sejuani, but that's personal bias talking) because his W in combination with Cinderhulk allows him to ramp up really, REALLY fast in the jungle now, since he gets 65 bonus health for every clear now (up from 52), not counting buffs or scuttle crab or lane tax. The way they buffed his Q and nerfed his E last patch also made jungle Sion even stronger, since you max Q in jungle, not E. I wouldn't be surprised if they made his Q cost 50 or 55 mana instead of 45 after this E nerf, because he basically never goes oom in the jungle even when spamming every ability on every camp.
Sion is very good right now and the little nerf they are giving him is warranted, and I am surprised their aren't more.
That said, I'm disappointed their are no Irelia nerfs this patch, because she is also really strong in top lane and has been for a while, where Sion is only just on the fringe of competitive until recently.