It’s a matter of taste… while I agree with you that it generates noise… I can “identify” the person by looking to their identicon better then seeing a letter and then having to read the name to make sure it’s the same person.
Like I said, it can be a option to choose which “identicon type” will be used… and one of them be the letters.
I believe that the profile can have the option to “upload an avatar” or “Choose a identicon type”, and the installation have a “default flag” for the identicon. What do you think?