Thanks R C-R, and Old Toad, I stand corrected. My apologies, and to Apple as well. I am suspect these days, given the headlines, when talking about security and everyday terms such as 'wear-and-tear' are used - as they seem to be convenient to placate the novices.
Below is my experience from running the command. I just did a 1 pass, but it took a LOT longer than when the feature was in Disk Utility. The error (69847) happened in Yosemite too (when I got my mac) so it looks like the same thing. Nevertheless, it worked as Wondershare's data recovery app found absolutely nothing.
<my>-MacBook-Pro:~ <me>$ diskutil secureErase freespace 1 /dev/disk1
Started erase on disk1 Macintosh HD
Creating a temporary file
Securely erasing a file
Creating a secondary temporary file
Mounting disk
Error: -69847: Couldn't create temporary file
Underlying error: 1: POSIX reports: Operation not permitted
<my>-MacBook-Pro:~ <me>$
But better than erasing free disk space is secure erase - which is also still available in Terminal, and explained in this article.