Updating the posts would involve the destruction of data. Making sure you only update the posts that have been updated, with zero accidental clashes were 2 unrelated posts had the same title, or if the post was updated in the import, and on the site, etc etc.

It's safer to simply not update, thus ensuring there is never a loss of data.

This question implies you have an issue syncing content between a production and staging environment, and this is your solution ( which itself has an issue ). Perhaps you should return to the original problem.

Updating the posts would involve the possible destruction of data, which would be unexpected and could cause a lot of problems. This is why the importer doesn't update posts. 

Making sure you only update the posts that have been updated, with zero accidental clashes were 2 unrelated posts had the same title, or if the post was updated in the import, and on the site, etc etc is extremely difficult.

It's safer to simply not update, thus ensuring there is never a loss of data.

This question implies you have an issue syncing content between a production and staging environment, and this is your solution ( which itself has an issue ). Perhaps you should return to the original problem.