As per the title,
I personally think that a system that allows for the freeplay session in between matches to be shared to so your teammates can drop in and practice with you is a fantastic way to create chemistry, practice formations, and just talk tactics together while not under pressure of a ranked match.
As for Custom Games;
I'm level 606 ( I probably play too much I know ) and I've made countless friends on Rematch. The use of custom games is very rare in my personal experience, no one wants to leave a lobby, create a ruleset, invite all their friends, and so on. I think (and hope) I speak for a vast majority of players, that simply being able to hop in a freeplay session with a single press of a button would be far better.
Hope this makes sense 😁
*Edited - said lobby but meant squad