Recently a media outlet focused on the ad world demanded a three-hour unpaid test to compete for assignments paying $25 for a whole bunch of research and high word count. Ha-Ha-Ha. The deal was listed on respected MediaBistro.com. Smart writers I know didn't go for it.
Also, there are scams. Organizations just want free ideas or words they can scramble for their own copy. After a while we can spot them.
But, sometimes there's enough money and long-term opportunity to take those unpaid tests. For example, an assignment for ghostwriting a $100,000 book. If the author whose name will be on the cover wants a few pages of the Introduction, we might consider investing in that.
In this ever-shifting marketplace there are no longer absolutes. We make our investments of time and hope on a case-by-case basis.