Frankly I don't care if it does result in spam. Spam is a secondary phenomenon of a thriving game world in an MMO. I'm not concerned with being a defensive architect who builds everything around the worst outcomes. The problem there is the spam, not the outlet. And if it's RMT then Square's job is to deal with them. You can't lockdown and stop making anything new just because there's going to be some problems.

Posting group requests outside the game itself is fine but impractical, and not even new. Those things have always existed with forums since FFXI started. And you all know they don't work because only a fraction of players will waste time navigating to read your request for help thread tucked away in some nook on the internet. Let's say that I'm bored, and I'm curious to know what groups are forming. I flip my server chat filter off and go out to solo subs in the meantime. Now I can do stuff without idling in the city. Then if there's a group forming for something I need I can reply and get info right away.

The advantage is that a chat message is immediate. You see it and you know this person is looking as of right now. You are both there and ready in the game right now. When you post to a forum somewhere, will someone see it? Will it matter by the time they read it? FFXIAH has a nice bazaar feature that lets you show items you're selling and willing to buy. Some people do use it, but how much easier/more practical would it be to have instant messages in the game?

Proposing you make a server-wide "linkshell" type of deal is silly, as it accomplishes the exact same ends of one channel with an on/off filter. Except it needlessly complicates things. (Needless complication being something already overabundant in FFXI)