Pros:
Efficiency and instant feedback
Writing instruction is hugely time consuming. Even cutting down conference time to five minutes a student a teacher can be hard pressed to meet with all students regularly in a large classroom. Theoretically, a computer providing edits to writing between student/teacher conferences could give a student instant feedback on changes made on a daily basis. Any concerns the student might have based on the computer generated feedback could be answered during the person to person conference with the teacher at their weekly conference time. This could also streamline the efficiency of conference time.
Record keeping
If students are uploading edits into a computer system then edits can be saved and recorded to monitor changes. This could give teachers additional data to use during assessments and for parent conferences with very little extra effort on the teacher's part. The teacher can also quickly see if comments and suggestions were implemented by the student.
Cons:
Sample sizes
The biggest issue currently is that computers need a LOT of samples to draw accurate conclusions from. My understanding of machine learning is that there needs to be several samples for a singular question in order to begin accurately giving feedback. This would mean having multiple prompts would be very difficult to manage in a class.
Incorrect use
Technology like this can be use well or used poorly. I think currently a lot of our US teaching force is under prepared and unfamiliar with many different technologies. This unfamiliarity I feel has lead many teachers to not use new tools to their full potential. Some teachers may be tempted to use machine learning to cut their required amount of grading instead of as a classroom aid.
Student perception
I would be interested to see some studies about student perceptions of teachers using technology to grade their essays. I know personally, one of my concerns with this technology is that I would feel very distraught knowing that no one could be bothered to read something I'd put lots of effort into. This might make some students put less effort into their work than they otherwise would.
Of course there are other pros and cons. As usual I am leaving things a little more vague in the hopes that discussion with develop between my education colleagues. To read more about machine learning and writing check it out here.