Thanks for the early reply!

I wanted to test some things out in a custom game before I replied though so sorry about the delay (yes super minions do crush the voidlings if you them completely alone). This response is going to incorporate some other good comments I have seen so far so I also apologize for the length I want to expound on like what mdk_777 said because he really covered what I think about Zz'rots cost-efficiency. Cost efficiency isn't that important later in the game and the only other mixed resistances item is GA. If I bought GA (which is 10% less cost efficient) no one would think twice but Sion really prefers to die during the chaos of a team fight. I would argue other tanky champs like Mao are generally not that useful coming back into the fight after the rest of your team is dead.

As for holding a lane, the Void Gate does exactly that when placed defensively. Place it in one of the three bushes up in the top lane (or the tri- bush) and you know you won't have to worry about that lane being pushed to your side of the map for the next 2.5 minutes (this all applies to bot lane if the team fights are gravitating towards baron). In that sense it really just acts as an outer turret to keep yiur lane “pushed.” With the new trinket changes I like to upgrade my yellow early on and light up the top half of the map so my support can take care of drag. It’s really amazing how much vision low elo can have when 2 members have 3-4 wards on the map at all times, but I digress. I mention the warding because it's always nice to keep vision of Baron for your team and that tri-bush is right next to where you need to be placing wards every 180 seconds or so anyway. It can be easily punished in coordinated teams but I usually can sneak up from mid and return without much going down.
That being said, I think you bring up a very big weakness of the item as a “6th man split push tool.” The portal does have to be very close (golem bush or closer) to really take advantage of the offensive siege power. I think this might work against a clueless team up until you get to the inhib turrets, but the strategy would most likely have to be something you did only when you had TP available. It probably would work “well” in a generic 5 man siege depending on the comps also.