Silver and below I rarely see people use them. Are they used a lot more in high elo? Every other game or every game? I'm probably going to learn a lineup or two to help me rank up but I'd like some perspective of anyone in higher ranks

Let me start saying: there is no reason at all for Riot to not let us pick our agents on those two modes. Those modes are mostly gunplay based and having double agents is not a big deal. Some people can't play without the agents they are...