Allow me to set the scene for you.
Only once in a while, First Posts and Low Quality Posts are active, i.e. actually have some waiting to be reviewed. (it's a tough world waiting for reviews)
So, I click First Posts, which has 15 ready to be reviewed and I come across a post that needs a comment or two to give them guidance.
Then, when I click 'I'm Done' there are no more reviews queued.
So, doing the 'right thing' and spending time advising users on how they can improve, means I lose out on future reviews, whereas taking the easy way out and Recommending deletion, allows me to continue reviewing (only because I'm then quick enough to catch some left in the review queue).
In short, wouldn't this mean that reviewers are more inclined to Recommend Deletion, and thereby punish the new users, than comment and allow the new users a chance to improve?