I think that writing something down is best, then and now, for documentation purposes. Let it be recorded that I proposed, you declined, and a few of your reasons I need to address "for the record" and for the record, bless you.

Makes sense to me. What is bad about the written word, is that the person who writes and sends it does not have the opportunity to know the circumstances in which the word was received, or even if ever. (Lizzy could have torn the thing up and he wouldn't have known. Or a dog ate it.) Synchronous, in-person communication allows for immediate feedback on whether something is understood or not. So, I agree the letter could make for tension. But it also is best for Darcy who doesn't think Elizabeth will accept him anytime soon with a reception she gave him!