I think it adjusts mmr slightly, but I think a lot of the time that is just balanced by the other team having a duo.
As for reasons to do it or not there are a few I can think of each way:
Pros-
- If you tilt easy having someone you play w/ who has a good attitude may help you stay focused and not throw games.
- If you learn the other person's play style or have voice chat you will coordinate easier than random solos. (I.e. If your duo always goes ham you know w/out typing/pinging that they want to dive)
- A second person can compliment your play style if you match right. My most common duo partners had good mechanics and usually won lane, but didn't ping objectives much. I never won lane as hard as them but one game a team asked if I was sure we could do Baron w/ our hp and time to enemy spawn and my duo chirped up "I've never seen him judge one wrong so far, he's pretty good at judging Baron damage given game time and levels" and I realized part of why we did well together is I didn't win or lose lane very hard most of the time, but I knew how to push an advantage when my team did have the lead, they crushed lane but didn't always push that advantage when they could, so we each covered the weakness of the other one.
- If you duo with someone who is just better than you overall they can help carry you.
Cons-
- If you tilt easy and your teammate criticizes teammates there may be more arguments to put you on tilt
- It may be difficult to find a good duo out of random players/if duo w/ irl friends they may be a bad duo match for you. Ideally you want someone who prefers to play two roles that aren't in your two favorites, and their favorite champs should fit in team comps your favorite champs do. For instance a rengar jungle/top main and Ahri mid/jinx adc main would work well to be a pick comp w/ Ahri and rengar, or to get jinx excited at the start of a fight w/ rengar pick off and let jinx have mobility to clean up.
- If your duo is worse than you in all regards then you have to carry more on average.
Hope that helps. Sry for wall of text.
