For the same reason the site exists, which is to spread rationality. This seems like the default position.
If you disagree, I think it should be because you think "spreading rationality" is not the goal (perhaps LW exists as a place for a certain group of people to hang out?) or that the current size is optimal or too large for its purpose (which seems quite unlikely).
But... net increases in traffic that come from sending email notifications to people who are already participating in discussions doesn't have anything to do with the kind of "spreading rationality" which is to some extent tracked by view counts. Cf. Goodhart's Law
I think it'd be a good idea to keep a list of the ways we'd like to see LessWrong improve, sorted by popularity. Ie. email alerts for new responses.
So if you have an idea for how LessWrong could be better, post it in the comments. As people up/downvote, we'll get a sense for what the consensus opinions are.
I think there's a pretty good amount to be gained by improving LessWrong.
Note: I say "ways to improve" instead of "features" because "ways to improve" is more general.