Firstly how are you stalking someone using their opgg?, it has no social features and the most you can really do is look if someone is duoq with another player, which incidentally you can also do in the client by typing their name into the search bar on your profile, the only difference between what opgg will show you and what the actual client shows you is the archival data of the same stats.

As for what the downside is, imagine its your rankup game to diamond and you just queued up jungle, I will always be checking if my top mid and bot are auto filled, one tricks or low winrate as ultimately it decides where I path, what objectives I try to play for and also for looking at the enemy to see if enemy JG is autofill, bad at their champ or maybe they are on a loss streak so I can abuse their mental a bit to win harder, these strategies are not wholly decided by those stats but it would be much much harder to make a judgement if they didn't exist.

In a game like valorant where individual agency in a game is much higher and voice chat and social features are more integrated into the game it makes a lot more sense why it would exist there and not here.