Furthermore it's probably on the OS installation disk, so you don't download anything that specifically marks your installation of Apache at all aside from updates.

It's part of the base install for every Mac OS X computer too (under the guise of "web sharing").

I wonder what percentage of *nix Apache and Nginx users, where a managed package is available, still download and build from source.

I believe nginx users are more likely to build nginx from source than apache considering that nginx modules need to be specified at compile time and the more you add modules, the more your performance might suffer (unless this has changed lately).

This is probably why there are many nginx packages in ubuntu repositories (nginx-light, nginx-full, etc.).

I did/do respectively.