But they already are at a disadvantage! Someone shouting in Limsa selling a particular item will miss out on a sale because the exact person who would want it is in Ul'dah. That person fishing right outside of Limsa would never know about that shout or any other shout either. A city wide /shout wouldn't fix that.
As it stands, you're missing out on, at least, 2/3rds of the chatter that could pertain to your situation. And that's just from being in one city and missing out on the WTS/WTB or LFG chatter from the other two.
This is exactly what global chat channels would solve. If there was a channel for selling, and another for buying you simply keep both open up in a tab and between fights or whatever task you are doing, you flick to it occasionally to see who's selling or looking to buy what.
This argument about missing out on potentially useful shouts because you have a particular channel toggle off is like arguing against YG having a bazaar section on their website because you don't go there to make use of it.
If you don't join a channel, then you don't get to see the chatter. Simple as that. You're already missing out on a majority of the it already. Adding the ability to hear the chatter from the other two cities can only help you. And people will still use /shout, of that there is no doubt, in addition to global channels.
Because if I had the option of selling something to the guy standing right next to me, or waiting for someone 2 zones and a boat ride away to come to meet me to buy something, I'm selling it to the guy right next to me. Hands down, no thought involved.
And I agree that there are other features they could only help improve the game, but a global chat is not the disaster that it's been made out to be. I've seen it work brilliantly in an MMO before.
Are your objections to it because you anticipate everyone on the server going into the WTS channel and having idle conversation and gossip about the latest fashion and news, overwhelming the legitimate text from people actually trying to sell something?