Ah, good article. It makes all the same points as I, yet it still insists on drawing the opposite (i.e. wrong) conclusion! Here's another good article with comments. I generally agree with the contrarian commenter "Warsaw Will".

Here's my big peeve: People have gotten so accustomed to using "that" for restrictive clauses... that it's now, unfortunately, increasingly common to see people using "that" with non-restrictive clauses! E.g.:

The perl mongers meetings, that tend to run long, ...

Even worse -- far worse -- is the disgraceful tendency to go one step further and use a form of "that" when the sense is possessive, i.e.

The perl mongers meetings, that's attendance is on the decline, ...

Gag me.

And this is why I believe it is sound advice to recommend that writers entirely avoid using "that" for relative clauses, at least as a default. I would prescribe the rule of thumb thusly:

• Would you have to use "which" if the clause were prepositional? (e.g. "those with which") If so, then use "which" even if it's not prepositional.
• Would a word like "who/whom", "when", or "where" fit in the place (disregarding the semantic impropriety)? If so, then use "which", not "that".

The argument about commas, and whether the use of "which" sans commas could lead to a question in the reader's mind as to whether commas were intended but inadvertently omitted, is a straw man. We have no such issues when dealing with "who/whom", "when", "where"; therefore, so no such issue should be imagined with "which".

Incidentally, the term "which hunt" certainly has art prior to Rachael's 2006 blog comment. My copy of The Elements of Style, by Strunk and White, (dated 1979) says:

The careful writer goes which-hunting, removes the defining [i.e. restrictive] whiches, and by so doing improves his work.