Bungie it seems don't want you to play 'as a disparate group of strangers' so they are under the assumption that everyone will have a bulky friends list from the get-go with available players at that given time and shouldn't need to go fishing for buddies. It's a bad assumption by them IMO but they recognise their risk in doing that. Also, if you have to recruit via the tower precisely the number of players you are short to play; that kinda destroys their wishes in terms of playing with strangers regardless. MMO's are all about making friends in game as you go.
If some Raids are 4+ hours straight - fuck that. Who the hell (apart from kids) has that much time on their hands in one session? Not very healthy either if you can't have a break or save midpoint.