User blog comment:Deshiba/Pre-Season Theor Craft: Twitch/@comment-1330314-20131219172004/@comment-1330314-20131222002344

Champions like Vladimir or Orianna just need the enemy team to be grouped up for an instant so that they can land their ults. With Runaan's, the enemy team needs to be stuck together for a much longer span of time for it to work. With the enemy team and yourself constantly moving around, your secondary bolts become very unreliable even if they do hit enemy targets, as you're far likelier to hit the enemy's tankier front line than their squishy back line.

I'm not saying you don't have a team to back you up. I am, however, saying the enemy ADC will outpace you if you get a Runaan's and they get another marksman item, all else held equal. I would also not recommend maxing out your attack speed on Twitch, as his innate steroid means he scales far better off of attack damage (though you shouldn't always initiate with it, as it's also your escape). A Phantom Dancer is all you need to apply full stacks of your passive in a fight.

The problem with the layout you suggest is that it's very easy for the enemy mid to freeze the lane and prevent you from dealing any significant damage to the turret, as most mid champions can fend off poke and last for very long in lane by themselves. Meanwhile, your lone melee bruiser is going to get bullied bot lane, meaning the enemy duo lane is not only going to get more fed, but is going to be far more likely to take down their turret and start roaming before you do. The other problem is exposure: bot lane has one, at most two, points where the jungler can gank, and if there's only an underfed solo laner to help, their ganks won't work. If you decide to roam, you can only use those two points to gank. Mid, on the other hand, is directly exposed to the other two lanes, and has six points where the enemy can gank. Lane length is irrelevant: the enemy mid will be able to support gankers with crowd control or burst, and whereas you can retreat away towards the bushes in top or bot you can only move towards your turret mid, which allows the enemy laners to anticipate that and cut you off, or even tower dive you, giving them an extra two points of entry. Contrarily to what you think, mid is the most vulnerable lane in the game, not the safest, if only because no matter where the jungler is they can reach it quickly for a gank. Top is typically the safest lane, as it's far away from early jungle objectives and doesn't influence the game's outcome too hard if one lane wins over the other (unless one of the tops feeds).

I'm not assuming anything, I'm just pointing out that a lot of the assumptions you yourself are making (that the enemy's squishies are all going to be close enough together to get hit by your three bolts, that mid is a good setup for a duo lane, that said duo lane is going to win against a champ designed to resist against bullying while your bruiser holds their own tower against a ranged damage dealer and their support, that neither of these lanes get ganked by the enemy jungler and/or laners, etc.) are flawed and unrealistic. I'm not being closed minded, I just realize that there is an underlying reasoning to certain core parts of the meta which can help explain what is viable and what is not. Runaan's is never picked in high-level play because it's at best unreliable and at worst gold inefficient, and marksmen moved away from mid to bot lane because mid became more vulnerable, not less.