We all know that it’s important to avoid major grammar errors if we want to present ourselves in the best possible light, but it’s often subtle grammar errors that ultimately make the difference. Even when those subtle errors go unnoticed, they make the same difference that one off-key violin note might make to an orchestra. The average listener can’t pinpoint what’s wrong, but he knows that the performance wasn’t quite at its optimal level.
The incorrect usage of that instead of who when referring to a person is one of those subtle mistakes. John is the person that went to the store is incorrect, while John is the person who went to the store is correct. It makes complete sense to use who with a person and that with an obviously inanimate object, but sometimes, there are legitimate reasons for the confusion.
The media often uses that with a person, such as The person that robbed the bank also robbed the store because the media wants to dehumanize the report–they want it to be dry and factual, not human. While commonly used in that way, it’s wrong. A person, even a criminal, is animate.
There is also confusion about which to use when referring to animals–do they warrant a who or a that? That’s often a matter of personal opinion and usage. It’s hard to imagine that many of us would consider an ant to be a who; however, most of us probably consider good ol’ Rover to be a who. I’ve heard it recommended that named animals should be paired with who, and unnamed animals should be paired with that. In other words, both Whiskers is the cat who we all love and This is the cat that we all love would be correct.
Regardless of the instances of understandable confusion, there should be no confusion when dealing with people–while the point may seem debatable at times, we’re all worthy of being referred to as who rather than that.
–Paul
Paul’s book–Literally, the Best Language Book Ever
Everything Language and Grammar 
The that versus who thing isn’t a ‘rule’, it’s a convention. That in reference for people have been used for centuries. In BE, it’s common to use either who or that to refer to people. While the bit about journalistic de-humanizing was interesting, I don’t understand why people still jump to call the who-for-people-only a ‘rule’.
Some people say don’t use ‘whose’ for inanimate objects–it’s a rule, but it’s not; it’s a convention too, and not one that will be around much longer.
Why is it even necessary? How many people didn’t think of ‘that’ as de-humanizing until someone told them such, so to justify this non-rule?
Reply from Paul: Grammar rule or convention–it doesn’t much matter to me. Common sense dictates that a person, who is a who, should be referred to as one (a who), while an inanimate object, which is not a who, should not be referred to as one. If common usage is your only guide, though, of course, you will be content to use either in both cases.
You use the example, “Whiskers is that cat who we all love,” which seems to me to be incorrect because Whiskers is not the subject (I believe that “we” is the subject of the sentence), but is, rather, a direct object. I propose that “Whiskers is that cat whom we all love” would be correct and that “Whiskers is that cat who bites” would likewise be correct because there, Whiskers is the subject.