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.
"If you can, help others. If you can't, at least don't hurt others." -- the Dalai Lama