I think that it would be a handy thing to be able to have the option of just sending one message to a bunch of people instead of having to go to each user's page and giving the message individually. However, so that people don't use this for spam, using this feature could just be worth 2 AP. This would help for people who run clubs or people who just want to tell all their friends something without having to go to each friend's page.
This is sorta what the forum threads for groups are for though. It is encouraged that all announcements are made in the threads, so that only one post is needed, and yet many members can see it.
I don't realy know of any messages that would be sent out to all members that would not be spam. Even if it was worth 0 AP, all anouncments would mostly be "Join my club, Hi, To all friends I am quitting, Take a look at this thread, please report my Screen name because I want to change it, invite please, check out this recon armor... woops sorry I kind of got onto xbox live messages...
Anyways I do not know of many messages that would be worth sending to everyone that is not spam.
Carlie is right... For an announcement post it in the forums. And, of couse, spammers would make multiple profiles then spam on all of them... So sad...