It's not hard to learn new ranks for a guild that uses different icons than another guild, but it is annoying.
You might be settled already with a guild, but what about the players who have still not found "that perfect guild" that they are searching for? You are gonna make them learn new ranks all the time?
This could easily be fixed just by using colors...
The color would = rank, and the icons just represent your guilds choice...
Color1 + icon = ldr, Color2 + icon = co, Color3 + icon = members
The colors would be default, and you can't change the colors.
That way, it's easier to make something like an "upload your own icons"
However, I think the devs just don't want to have too much work for this, so default icons are fine for now. You can develop the system more later on in the future.