So when you view your profile you can of course see the "Last Seen" date but if you go into Manage Friends you cannot see when they were last seen. And if you have enough friends and are wanting to see if any are inactive / can be purged you need to click into their profile and see if their active. This posses a problem if you are doing more than a few because you'll get a flood warning if you do too many. If it pulled their last seen date and stuck it in that info it would remove the need to tunnel into their profile and you can just check the ones you need

My two cents