This probably holds for vandalism-type issues, but not for access control.
Imagine the wiki pages about a pending layoff. In a large organization you might want 20-30 people working on layoff related content, but you wouldn't want your entire staff to have access to it.
There are also issues with corporate intelligence. When you're releasing a top-secret kill-the-competition product you don't want your temporary employees, contractors, janitors etc. to be able to read all about it. Likewise putting staff's names and skills online could be a treasure trove for headhunters stopping by for short times at the company.
Mediawiki doesn't have the functionality to deal with these scenarios.