This would break immersion, IMHO. Not like there have been to s of skirmishes between fighters Nd caps but in fleet fights everyone should be able to shoot everyone. It would lead to quite stupid standoffs in teamfights when one side one with caps, other won with snubs, and inRP the outcome does not make sense. Yes, it sometimes happens even now but at least there are prospects to win. With mutual invulnerability these prospects would not exist.