One thing that is nice about add-on mirrors is that you can then have one mirror aimed down towards the tires of the trailer, which is good for seeing if you are going to hit curbs or obstacles while turning. When I had add-on mirrors in the past I usually aimed the inside (stock) mirrors down a little bit.
This is what is really nice about having power mirrors - I can easily change their angle when parking the trailer.
Large "towing mirrors" are really nice to have, even if you are not towing all the time. You don't realize how much you use them until you take them off after using them for awhile.