That said, prepending pretty much anything more than your current view
of the Internet's diameter in ASNs is useless in practice.
That is one way of viewing it. But prepending can also be used for traffic engineering. I could prepend 1 to my free peers, 2 to my paid peers, 3 to cheap ip transit, 4 to expensive ip transit etc. The linked draft RFC does not appear to discuss this at all. The depth of prepending used this way only relates to how many different classes of peers you can imagine in your setup and is not at all related to the "internet's diameter".
To someone on the other side of the planet, who are neither peers nor customers of peers, they will just observe that I am prepending 3 or 4 times and wondering why the extra prepends? The answer is that closer to my home there are people who are observing the same routes with 1 or 2 prepends and that it matters.
The draft RFC lists some alternatives to prepends of which none can do anything of the sort I just described.
Regards,
Baldur