Point taken, I won't go so far as to say that the 5.7 and 6.4 will react 100% the same way with 87. However, if you can find a bunch of guys saying "I hear pinging in my 6.4 with 87" then it will be happening in yours too.
Sometimes it's not enough for us to rely on our ears/senses. A computer/logger would be able to show how badly these engines ping on 87, and even 89 isn't all that good. Engine pinging is not a good thing to have happening, so the more we can reduce it and/or the chance of it, the better it is.
A lot of us spend thousands on mods or toys etc, I always feel a little baffled when I see them arguing against a higher octane, logically it doesn't make sense to me to run the cheaper stuff but I guess we're all different with different spending priorities.