While I'm mostly in Camp Inclusion, I appreciate the issues that tend to come up as you loosen criteria for articles more and more--you probably inevitably get articles that are "notable" to a narrower and narrower set of people and a lot of verification depends on shakier and harder to access sources.
That said, it's pretty clear that there are more than a few Wikipedia admins who seem to have embraced deletionism for topics that aren't near and dear to them personally and/or which poke at whatever their particular political hot buttons are.
The nice thing about Wikis in general is that it's usually easier to correct a misuse of the power to edit a page than it is to commit the misuse in the first place. That's why Wikis work at all.
I think the general unspeakable reason is pretty simple: once you give people power, some of them will misuse it. And since it takes more effort to correct a misuse of power than to commit the misuse in the first place, any institution that gives people power becomes more and more corrupt over time as misuses of power outweigh valid uses of power.