that/which

Barbara Wallraff's Word Court new column is dedicated to a reader's query about when to use that versus which, prompted by warning from Word's grammar checker. Her advice is interesting on a few counts, starting with the comments about how grossly unreliable that grammar checker is. (It's not good enough to be worth an undergrad paper figuring out what mistakes it's making, as far as I can tell.)

But she goes on to write that:

Because the rule can be tricky to apply, it's hard to fault people who choose not even to try. Well, if the rule doesn't come naturally to us native English-speaking people, asking a grammar checker to follow it is asking a lot.

Well, it's an artificial rule for lots of us -- without any real connection to how we learned to speak and write, certainly in the classic 'restrictive' versus 'non-restrictve' clause cases. She then tries to thread the real usage versus prescriptivist needle:

I hope you will keep making the distinction between "which" and "that" -- partly for the sake of paying attention to what you write and say, partly for the sake of your audience, and partly for the sake of the language itself.

We're all with her on the first point, and you could argue the second, but the third point could lead me to campaign for bringing back the dative case, maybe the instrumental.