Honestly, I have yet to hear of a linkshell on Aegis that has this rule. I run my own linkshell, it was social at first, but lately we've been scheduling events and such. I don't consider us an endgame linkshell, but there is no way... ever, that would cause me to enforce a rule like that. It's just ridiculous. In XIV, multiple linkshells are kind of needed, no matter how full and active your list is. You can't take everyone with you to your 'scheduled' events.
That's my opinion anyway... some rules are just too elitist for me. I tell my linkshell members to join other linkshells especially since timing can be an issue, and they are still pretty active in mine.