An End to Pedantry: THAT vs. WHICH

I've used this blog on a number of occasions to debunk rules that are widely believed to represent 'proper' educated usage but which have very little basis in reality. I have for instance argued that data is not in fact a plural, that tomatoes aren't really fruit outside of botanical circles, and that there is no good reason to use an rather than a before words beginning with h. I'm far from alone in writing about these imaginary rules but I occasionally feel I can provide a slightly different perspective on them. After encountering some of my posts on these issues, one of my readers asked me to elaborate on what I meant by this:

The only standard by which we can judge the use of a word to be correct or not is whether it conforms to the conventions used by members of the speech community in the particular context in which it is being used. [On falling apples and whether tomatoes really are fruit, Oct 16, 2013]

In particular, this reader was interested in what I thought about the alleged rule for when to use that and which, and whether I think it actually matters whether we follow such rules if our meaning is likely to be understood in any case. What follows is a slightly edited version of the response I gave this reader by email, which I hope clarifies my position.