I know the cost and the such, but It just makes more sense to me that the person closest to both lanes wards? Especially with the Rift Herald dropping in next season.
I mean everyone should 
ward
, but I feel like Mid and Bot should switch.
Is there a reason besides "tradition" that lanes have kinda stuck? :)
The solo top, one jungle, solo mid, duo bot meta has been in force for awhile. AD carries are very weak early and scale only with gold, so they need to be able to last hit well; they have a support (who has enough base utility to survive on very little gold) to help them with that. They're placed in the bot lane because botlane ganks can turn into dragons fairly easily, and having a support makes it more likely that the carry can survive a gank on the longer lane.
Although AP carries (the typical midlane role) need gold to function well, their most important resource is experience, since ranking up their abilities gives them most of their power. With that being so, they're placed in the middle lane; since it's shorter, it's much harder to be zoned out of experience range, and the shortness of the lane also gives them a decent chance of escaping a gank or all-in even without a support to help them. Since levels are their most important resource, giving up a whole wave of XP to ward the river shouldn't be common enough to require a sight stone.
If you're looking for another sight stone on the team, jungle is your best bet. Although junglers don't earn as much gold as laners, their items are also remarkably gold efficient, meaning that they can pony up without too much pain. They also spend a lot of time roaming around gank paths anyway, and so get full use out of the sightstone without roaming too far from their intended path.
The reason mid laners don't buy sightstones is that they aren't as capable of running on as little gold as supports or junglers are. One of their jobs is usually to apply damage, and those roles can't really be shelling out the 800g and setting themselves behind or wasting an item slot.
And as for mid being in charge of warding, that's also a less than optimal idea. Yes, they are closer to both lanes, but since they have to keep up their farm, roaming isn't desirable unless it can net you a profit.
An upgraded trinket is all mid needs.
If mid got a sightstone item. It would be a must have on mid laners, and if the item didn't work on your mid lane champion of choice, they wouldnt be viable. And if the item had stats to compensate every mid pick (which isn't possible) it would be too expensive and they wouldn't get it anyways. So nope. they shouldn't get a sightstone.
