Let me start by saying: No, I dont think we need that.
Now to conter a few of your arguments:

- if a player is new or doesnt has a hunting-LS but becomes intrested in that part of the game nothing is stopping that person to get themself out there, find a hunting-LS and become part of a community that is actively hunting - as in: Looking for hunt-mobs aswell as spawning them (S-ranks have special conditions and often active hunters are trying hard to fullfill them in order to spawn that S-rank). That new person shouldnt expect to get that sort of thing handed to them - if they want it, they might aswell put the effort into it. Also: Whats the point in a new person who just reached level 50 to hear of an S-rank in The Lochs? Or the Ixion-Fate?
- Hunting-LS are excatly the place to work together towards a common goal. A global chat isnt. You dont develop a friendship because someone posted the position of an S-rank and you rush there and shout for an invite along with 100 other people
- How do you limit the global chat to Hunt/Fate-Callouts? I dont know if you've turned shout-chat off but with the amount of FC-advertising-spam we're getting in there already I really, really dont want to see a global chat with even more spam of that sort. (Okay, I could just turn it off for myself - but I know a lot of people who'd do the same and if half the server turns off that chat anyways whats the point in it?) I guess you could try something like a posting-limit for the global-chat but nothing would stop people to use theres up on, well, FC-spam or other nonsense.
- Hunts and Mega-Fates arent supposed to be done with the whole server. Zones cant even handle that many people - I dont know about your server but on Odin we still have instances for Ixion-zone to prevent it from crashing. Those arent events that are supposed to bring the whole server together.
- HP of S- and A-ranks are always the same, they dont scale. So if a party of 8 can defeat a hunt-mob on one world, a party of 8 can defeat it on another world - unless that party isnt skilled enough. If it takes you guys on Famfrit more people to kill something than on other worlds, that might be a problem with a general skill-level - no insult intended, but that argument is downright silly, sorry.

You're talking about forming bonds and about having social interaction to keep people playing, but your suggestion seems like a horrible way to do that to me.
You dont form bonds because you follow a shout into an area to zerg down a mob in 20 seconds.
You dont have anything to do because you're waiting for those callouts, made by people who are actively hunting, looking for mobs, even spawning them.
You can accomplish both though by joining a hunting-LS and become part of that community if thats what you're intrested in.

Sorry, but a global-chat, specially in this context, sounds like nothing more than heaven for FC-ads and hunt-"leechers".
(and seriously: There is a lot more to do than MSQ aswell - hunts/massive Fates are just a small part of that - I agree on the social interaction part and maybe this is just me, but I can only repeat: I doubt that a global-hunt-callout would accomplish that)