Black cleaver changed the way people build tanks- more health, less armor and MR. And while MR items are more diverse now, it doesnt seem like they ever get stacked to the point that a void staff is ever more effective than, say, Sorcerer shoes or Liandries Torment.

Does Void staff serve a purpose anymore? or is it a relic of physical/magic symmetry?

Uhh
It still goes through Magic resistance as it always has, and goes better with sorcerer's shoes and haunting guise now...so yes it still serves a purpose and is staple on most AP casters...

It basically makes Sorcerer Shoes, Haunting Guise and its upgrades useful to Xerath now.

But seriously, it's fantastic when they build MR, and when they don't, you have Liandry's/Blackfire and Deathfire.

That's the thing... WHEN do they build MR?

Oh god, Xerath with Sorc+Guise+VS would ignore all the MR

if they dont build MR then why would you be caring about this item?

plus if they dont build MR that should mean you have an easy time nuking them and thus your winning the fight, then if they were smart enough they would be building MR when they noticed they cant even survive a single volley of your attacks, then at that point void staff has a reason to exists

This item is like LW, but for ap champ. Required when they get MR, optional when they arent.

This item is build 4% of the time in ranked, as of lolking.net. Why so low? Considering that 2/10 are ap mid and that there is some games where they aint any ap, thats 20%, so that mean 1 build on 5 (ap mid) got voidstaff. And void staff is a late game item, so there is games of 20 ,imute when u dont have time to buy it. That would leave 1/3 of ap mid build wich would buy void staff. You cant say void staff isnt use.

It's good for negating large amounts of MR just like Haunting Guise is good for negating small amounts of MR. The season 3 changes means you can get both and upgrade Guise so you don't set yourself behind by getting an early haunting guise. If they don't build a lot of MR, then skip it for more ap/utility/defense items.

I don't think BC/LW will have the same punch on Brand as a VS