is this sentence correct?

[Andrew Whitehead]

I think in this case 'which' and 'that' can both be used with a defining clause.

Grammar is always changing and the fact that so many people do use both means that is has or will become part of the 'officail' grammar rules.

That is a debate that will run for ever, and there will always be people who argue that grammar and dictionaries should reflect use rather than prescribe it.

It may be true in some cases, but in my personal view of that argument is that it can be condensed to 'if enough people do it wrong, it becomes right', which to me doesn't make sense.

The bottom line is that using 'that' for defining and 'which' for non-defining is not dificult, so where is the benefit in incorrect use?