Who that? (And other non-errors)

Over there on Facebook came a very good usage question:

http://public.wsu.edu/~brians/errors/adopted.html

In the post linked above, on what I'm assuming is your website, there is a sentence that says, "Adaptive parents would be parents that could adapt themselves to changing circumstances." Regardless of whether the website is yours or not, could you please explain the reasons for referring to the parents as "that," instead of "who"? I learned that people should always be described using the word "who," and that only objects should be described using the word "that." Either this is incorrect information, or there is an error on this web page.

And I was a little surprised that I could not find this addressed directly on the main page for Common Errors in English Usage site. That was not a problem, though, because it was simple enough to find authoritative information, with references, on other sites. I replied:

I was also taught that at some point, but people that teach it that way are not informing you fully. "Who" in such situations is never wrong, but "that" is also correct. Here are two excellent explanations:
http://www.copyediting.com/it-s-all-relative-using-and-who-correctly

and

http://grammar.quickanddirtytips.com/who-that-it-he-she-people-animals.aspx

If I had been thinking straight, though, I would have remembered the Non-Errors page of the Common Errors site. A quick scroll down takes you to this explanation:

Using “who” for people, “that” for animals and inanimate objects

In fact there are many instances in which the most conservative usage is to refer to a person using “that”: “All the politicians that were at the party later denied even knowing the host” is actually somewhat more traditional than the more popular “politicians who.” An aversion to “that” referring to human beings as somehow diminishing their humanity may be praiseworthily sensitive, but it cannot claim the authority of tradition. In some sentences, “that” is clearly preferable to “who”: “She is the only person I know of that prefers whipped cream on her granola.” In the following example, to exchange “that” for “who” would be absurd: “Who was it that said, ‘A woman without a man is like a fish without a bicycle’?”*

*Commonly attributed to Gloria Steinem, but she attributes it to Irina Dunn.

In any case, the point remains the same: Slavishly using "who" to refer to people will not be wrong, but "that" is also acceptable. People that insist otherwise are less than fully informed.

One last point to clarify: To address the "on what I'm assuming is your website" part of the question, the Common Errors in English Usage site is written and maintained by Paul Brians, while I, Tom Sumner, maintain the Common Errors in English Usage Daily Entry blog and the associated Facebook page. When you see replies to comments on the Facebook page, please do not blame Paul for those. On the blog you are now reading—to which we both contribute—you need to check the bottom of each entry under "Posted by" to find out who wrote each piece. Our usage advice is very nearly perfectly aligned, but there can be subtle, mostly trivial differences on controversial points.