So my point of having their names bold in tab is to make it easier to see your friends. Imagen if you have a party with, let's say 3 players. You just won a game and are joining a lobby. When I join a lobby I always open up my tab to see if I have any friends in the lobby that I can say hi to and basiclly just be polite to. If they are stone ranked is way harder to see them in tab, but with their names bold would make it a lot easier to see them so I can be that polite guy saying hi to my friends.
You can also be the even more polite guy saying hi to your friends in /fmsg. I guess if they're really your friends you should message them when you/they got on ;)