Crossrealm BGs assume a new opportunities and challenges for all players from different realms. It means you may face new opponents, learn their new tactics and PVP style, they may learn something new. The main idea is diversify PVP with other people and get new level of competition, because sometimes it's boring facing same scrubs again and again even despite of playing on realm with 10k+ population.
Yes I do. At least they may learn how to play tactically and stop being cucked on middle while farming kills instead of follow objectives. Do you think is it funny to play with some mega BWL equipped pro PVE players in your team who have no idea what to do on BGs and who behave themselves like a bunch of blind kittens?
And the last, you have a less chance to face a prem while play as a pug on crossrealm BGs.