This article is specifically about not being able to include some of your work in your portfolio, and this is not a problem that's unique to video game artists. It's true of all sorts of people in the entertainment industry -- writers, actors, and so forth.

But in reality it's not really a problem. Sure, you absolutely cannot take past contracted work that never saw the light of day and put it in a public portfolio without permission. Rightly, no team wants any part of the creative process leaked, except when the people at the top choose to.

But this still leaves:

1) A public portfolio of stuff that was actually released by the companies (final video game clips, movie clips, etc.)

2) A public portfolio of personal material -- projects of your own, designed to showcase your talents at their absolute best

3) A private portfolio of unreleased stuff that you can selectively and judiciously show in private interviews, when it's not competing (e.g. show your Dove soap ads to Snickers, don't show them to Dial soap)

And that generally works quite well. Especially your personal portfolio of material (#2) -- everything you work on professionally is often compromised by top-down direction. But when you create stuff that's 100% yours, it allows you to show your unique gifts in the best possible light.