I generally find it easier to find the friend for pm by finding a recent post cause when i click on a friend's icon nothing comes up to say send pm.
I see no reason to have friends on this site but its nice to have them anyway and its flattering to be asked.
I think its because of the way this site operates that they are not really necessary here.
I wouldn't say its a weakness of the site though. I have another forum which uses the same forum format and i find the same thing over there.
Where as on my photoblog site, friends are absolutely necessary to be able to follow and keep in touch with other members who's photoblogs i am interested in. Otherwise you'd never find them ever again.