Let's look at what happens when you take Teemo support without Sightstone
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You have just handicapped your team, you will be providing zero extra vision on top of the wards your team has as trinkets. The support on the opposite team will be warding key choke points and you will not be able to.
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You as Teemo, are one of the squishiest champions in the game, what happens when you eat a Thresh hook? You die Blitz hook? Dead Braum procced stun? Dead Brand lands stun? Dead
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You probably took ignite, now this is cute for the first 5 minutes, but then what happens when they exhaust your ADC and you ignite theres? Your adc does no damage, your adc dies, and then you die.
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"But Teemo can blind the enemy ADC", Sure, I'll give him his cute mechanic that can remove an ADC for 1.5 seconds. But what is any decent ADC going to do when they get blinded? Kite, and walk away until the blind is done. Do you know why they could just walk away? Because you are playing Teemo, and you have no there CC abilities unless they run into a shroom of yours.
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CC, you bring almost none, yes, I realize that after 20 minutes you can start spamming shrooms in a bunch of places and sometimes they make plays. But do you know what an experienced support is going to do? Red trinket the hell out of areas that are obvious to have shrooms in them.
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"But Teemo does a lot of damage early game" Sure, I'll give him that, he does do a decent amount of annoying damage along with the burst of thunderlords and his Q, but what happens after 25 minutes and you get hit by an Ahri charm? Or a Syndra stun and W? You instantly die and the fight becomes 4v5. You are absolutely worthless in team fights if the enemy team has half of a brain.
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Because you went full damage, you will likely be hiding invis in certain parts of the map waiting for a champion to come close to you so you can hope to one shot them, sometimes this is in the jungle, and I've seen Teemo mains sit for more than 2 minutes and miss out on a level of EXP because of it. Congrats, you are now behind in EXP.
So, just wanted to say, thank you to all the Teemo mains and Teemo players, and Teemo supports without sightstone that have given me 5-7% of my losses.