In the reject column, we even can see that a reason is given for the rejection. Some requesters just use an "X" to reject in the same way they approve. This is really a bad idea. These are some posts from today from a requester review site Turkopticon.
If you want to communicate with a worker, putting a comment in the approval column will probably never be seen. This is a screenshot of the worker dashboard.
With over 24K submitted HITs, it is highly unlikely that the worker would ever see any message left in the approval column. But the one rejection, the worker will definitely take a look at that.
Here is the message left with the one rejection on Jan. 19th.
The feedback column is shown on both rejected and approved HITs, but the worker is unlikely to ever see feedback on approvals. This feedback on the rejection was unclear and did not help in improving work quality. I am not advocating rejecting workers in order to communicate with them, because you can do that through the API and also through the bonus feature in the GUI.
If you do have to reject, give a detailed reason for the rejection because neither worker nor requester will benefit from an "X".