If SE disabled cross class abilities then i think classes would technically be more balanced than having access to skills that are not native to their class.
I'm looking at you sch with swiftcast summon. Without eos/ selene sch would get rekt.

1v1 would probably more blacklist space as ppl would get picky on who to fight even if there was an option toturn off requests.
Get spammed in shout for declining a fight, get spammed by tells " 1v1 me bro " .
Maybe SE will add it in 3.0+ when they get their pvp act together lol