Maybe that's why I didn't quote the whole message? Hmm..
Okay then, let's go all out and be super focused on the subject.
How would the ranks and colors be sorted? White is the bottom, and black is the top, or would it change guild by guild? How would anybody outside of the guild remember/know what color of each guilds' ranks are? I mean, we should obviously know, because what would be the point of displaying guild rank to outsiders then? If blue>red in some guild, and in other blue<red. There would be no unison between the colors and guilds. That would just create confusion, and people probably would stop caring about the tabard colors (ranks of random guilds' random members). Which I suppose is the opposite of what the OP was thinking. So we would circulate back to the fact that those colors wouldn't really mean anything. Also, when was the last time you saw somebody equipping a guild tabard? I don't even remember.
There are already many commonly used addons that display guild ranks and whatnot on player tooltips. Do we need such (minor) addition to the game.
If ranks were on active display, I can only imagine they would be one source for insults when somebody doesn't play as you want, and decide to bully player(s) based on their rankings as well. More fuel for the ever-burning fire I guess.