Why do people assume you randomly send friend request to someone you saw in passing when this topic is brought up?
There are plenty of reasons why you would want someone on your FL, them being a likeable person you often chat with is only one of many and definitely not the only valid one. People who don't have their crafters leveled/ suitably geared to craft 3* reciped often have agreements with crafter to send them mats + surplus mats in exchange for crafts. You can't mail something to people not on your FL.
People who hop a lot into PF will inevitably meet people with whom they want to play again. LS are well and good but people react much better to tells.

Just because it's called "friend" list doesn't mean it must be reserved for the exclusive club of close friends. With CW whispering and mailing still limited, not having someone on the FL is often simply not an option.
And I see no reason why notes for the FL should not be added. It's a QoL change that will most likely only require a very small amount of resources.