To be fair, not being able to make use of a keystone is hardly a thing unique to manaless champions. Look at Garen and Aftershock, for example. He literally cannot Trigger that Keystone. I understand it’s not a 100% comparison, but not all keystones or runes are meant to be used on absolutely everyone.
And though I know that I am probably a bit rare, I can at least give you a personal example of an applicable context for such a rune: Kennen. Yes, he has an on-hit attack speed build that works wonders on him. Thing is, not only do I not personally like that build, but I’ve personally found more success building him as an AP mage.
Now, I will preface this by saying I haven’t played him since the AP item update. However, having played him back from the days of the Tome of the Ancients days, the number of items that he could plausibly take in any given situation without automatically sacrificing gold efficiency kept getting chipped and chunked away as the seasons went on. About half the items are objectively worse choices in any given situation simply because of the fact that I’m playing for stats I literally can’t use. Meanwhile, any mana champion could build any of the items I build on Kennen without any inherent penalty.