I like the /say command, it saves me keystrokes. When I want to help someone on a Global Chat channel with a slash command, I can just type "/say <whatever>", no need to type out a full /send for the name of the channel, or add filler words at the start, or add a caveat about putting a slash at the front at the end, just the exact text you need to put in the chat box to properly use a slash command.
Plus, /say is used in the default bind for your character's battle cry, you can't just remove the command from players' use without coming up with some workaround to still be able to send your battle cry to any chat channel you want.