Why isn't it possible to gain a delegation by proving number assignment? Because your ISP can't be bothered. Is such unbotherability legitimate?
these years, it is one of the things when considering a provider from which one gets address space. part of the problem is that this used not to be the case. "rdns is not really useful" was the common thought. so many isps did not pay it much attention. now, more and more services are using rdns mapping to defend against crapola. so it has become useful, and quite needed in some cases. but it is notalways easy to justify to management the costs of cleaning it up, often involving your provider, sometimes your provider's provider, and on up the chain. i have three /48s from an upstream. rdns is delegated, whew! but they are not dnssec signing. this has yet to cause pain, but i expect it will somewhere down the road. randy