I think it’s depend on your operators and current level or advancement.
Veteran and beginner won’t have the same needs.
Veterans use friend support only for daily quest and friend points and for event, they want a specific operator they lack or not enough advanced.
My friend support is for friends not for me only , it’s one of the reason why i won’t rotate my list.
It’s supposed to be the most powerful, advanced and useful operator for others.
On another hand low level players are addict to some operator i guess.
They seek friend with an operator that they like or help them a lot.
They need powerful operator for farming, Annihilation, clearing a map with 3 stars etc.
They don’t have enough operators and they are under leveled.
I let my gummy E1 60 because it will help low level friends without E2.
When you don’t have enough friends in your list, you will be happy with a friend with constant operators.
In my opinion SA skill 3 and Eija skill 3 are the best candidates they are powerful and easy to use and understand.
You should have at least one, in your support list.
Sometime i like to try others operators and find new gems like this Shirayuki E2 in my friend list, because i don’t have enough resources to develop all operators at E2 90( and i want to develop my favorites too) . :)