You're joking of course, but that likely won't do anything useful.
If it's tracked, then ignore has no effect. If it's not tracked, then you might as well use .git/info/excludes which is pretty much the same thing but not tracked, or you can use a global excludes file, like ~/.gitignore is common (you have to configure git to point at it, iirc).
It _could_ make sense to ignore the .gitignore if some other tool is parsing and using that file, but that pattern is...troublesome so I hope not.
No. You never checkout a site directly from git to begin with.
You don't let other people know what files are ignored from git doesn't mean people cannot access them. :/
It doesn't even mean it's not in source control. It just means that IF it's not in source control it won't be added to a change set automatically. gitignore has no effect on files which are already tracked, and even files which are not currently tracked can be explicitly added.