Which role would you want the "worst" player on your team to have?
Actually tried this out with a Skype buddy of mine that's SR level ~10 with a group of me and other level 30s.
He did really well with Tahm Kench Support. Pretty solid with Amumu support as well.
Not so much with top, mid, or ADC.
As a support main I can certainly appreciate how much of a difference a good support player can make; you can snowball lane and send 2 people ahead rather than 1, then transition into a crucial initiation/counter-invitation roll late game.
But if you have voice chat you can generally get your buddy on-board with ~70% of what they need to be doing as well as teach them new mechanics like how to freeze lane, play passively, which minions to go after with Brace, how to coordinate attacks, who to focus first, and when to strike: allowing them moments to shine when they're otherwise at a serious skill/rune/mastery disadvantage.
With trinkets, max ward per player limitations, and all huge boost in free gold for a support you don't really need to be that on top of things to make a difference.
But throw a player at a huge skill disadvantage in top, mid, or ADC? you're looking at serious issues and generally a 7-1-4 carry, along with the poor guy having 1/3rd the CS of his opponent.
A few seasons ago it was top lane as you were stuck on an island vs. a tanky derp, but with all the carry top-lanners that can run there now, teleport being more prevalent and really favoring strong map-awareness, and the current tanky DPS itemization placing top-lane carries on equal footing with mid and bot-lane carries, you're going to have issues when someone starts loosing top lane to a Riven/Fiora/Jax/Darius/etc.