When people fight for the inclusion of websites and webcomics (for example) it's more of a vanity issue. It may be verifiable and presented in an NPOV manner, but the fact they're trying to get a comic/site with a tiny audience into a general purpose encyclopedia is POV too, an attempt at promotion.
Surely third-party verifiability takes care of this one. If someone else cares to write something verifiable about it, it should get in.
My point is that far too often, it's not "someone else" doing the writing. Hence the use of the word vanity. We can be the most complete encyclopedia in the world without taking all of the incoming drivel. If there wasn't cut-offs then we could list all scientists (they're mostly listed on their university website or publication) it wouldn't be vanity because they didn't write it themselves, but if the scientist in question hasn't done any noted research we would only sooth their ego. If we didn't draw a line near professors, should PhDs be included, and if those are included we might as well start on students...
Cut-offs are a healthy thing. There may be exceptions to the rule, but when sufficient information is included in an article it's easy to establish something needs inclusion.
A band may not have done any tours, no albums or charted hits out and not have any notable members or awards, but if they appear on Oprah most people would agree they deserve an article regardless of [[WP:MUSIC]] regardless of missing the WP:MUSIC guidelines.
It only gets problematic if people fail to provide enough information in the first place. You can't expect AFD voters to spend more than 10 minutes of research on their argument. In such a case simply expanding and providing such info to the voters is much more productive than arguing against deletion.
--Mgm