If the leaver leaves early enough, you can have a major advantage on your hands.
There's less exp/gold to split. If your team takes care of it right, you will have a strong power advantage till about 34 minutes in. Not sure if this number has changed since all the exp and gold changes, but it's certainly a thing.
The only rough part is that they will have superior map control, so long as they do that right. And if they do it right, which is rare, they can stop you from ending the game with your mid-game advantage.
Of course, the entire way you play the game has to change. If your mid-lane DC's, you don't just shove the jungler mid and just play like you don't have a jungler. You're wasting all those jungle camps and providing infinitely less pressure. You need to rush the forget out of a tower or two, force laning to end, even if it means giving up one of your own towers.
That was the worst part.
Every lane won. Our toplaner/jungler (? not sure which he was gonna do) just left during champ select and never entered the game. I was sup so I went top. Even I did fine in lane. Botlane won pretty hard, mid did great, and our present jungler did really well. Score was 12/1 by about 17 mins. From there people started making really dumb decisions, but enemy team were bads so it was mostly ok. They started to get into the game, and then there was too much arguing. We could almost win teamfights, and occasionally actually did full-on win teamfights. We once even got baron without having a smite.
Our adc was fine, but didn't contribute much at all in teamfights or would just run away. We had the opportunity to flat-out win the game and he just walked out of the teamfight and left me to die.
Then we lost.
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