While I don't really see the point in yet another, probably rather sparsely populated forum, I think it's perfectly valid to do so if one wants to. Like you pointed out yourself: This is the Armbian forum. There are people, however, that don't run Armbian, don't want to run Armbian, and might even disagree on certain design choices it makes. That also is perfectly valid, and as long as improvements made are flowing back into Armbian maybe even beneficial. I think it stems from a rather narrow view on things to outright disregard people that want to try and take a different approach.
As for my own Kernel repo .... that's pretty much for my very own and private use and for the not at all aimed at the general public Gentoo based rootfs builder I'm working on, mostly for work. If it is not appreciated to let things flow back into Armbian, that's something I can perfectly live happily ever after with.