"Some of these devices are viewing adaptive websites with a container width of just 60%…I’m sure you’ll agree that that’s pretty ridiculous."
No, I don't agree. These decisions should be made based on the layout, and how it can be best shrunk to accommodate smaller screens. Sometimes that might be adaptive, and sometimes that might be responsive, but neither is a "one size fits all" solution, if you'll pardon the pun.
Yeah, I’m with you on this. A design’s goal shouldn't be to slam content into 100% of the browser width. If you use fluid widths without sensible maximum widths, you’re likely to end up with really wide lines that are difficult to read. As an example, I like Designer News’ width, even though I’m currently reading it on a far wider display.
“As you can see, the adaptive approach has rigid containers”
I’d almost go as far as saying that’s a better approach. At least that way you have some control over exact layouts and can tweak widows and orphans. Most fully responsive sites look bad at some widths. Do not want.