I think you would do the same with a ketch. Yeah, there are many ketch and yawl owners who do not set the mizzen. There are at least as many sloop owners who do not set any sail at all, preferring to motor instead. Often there is no accounting for how people sail their boats.

The mizzen makes it easy to set the main, as it keeps the boat's head into the wind. I can heave to in normal conditions at about 30 degrees apparent. When reefing, again the mizzen keeps the boat stable and headed up. At anchor in a field of sloops when it is blustery, there is no comparison, the mizzen keeps us lying within a few degrees of the wind while the sloops are tacking through 60 or 90 degrees. For all those reasons, I usually put up the mizzen as soon as I leave the berth, and it may stay set for many days of coastal cruising and many nights of anchoring.

If the mizzen is well separated from the main, it will add noticeable drive, even upwind. A ketch or yawl isn't thought to be as weatherly as a sloop, but on my boat with the small mizzen well aft of the main, it makes no difference in pointing angle and adds a couple of tenths to boat speed. Upwind I do not have to carry the mizzen boom near the centerline.

And backing out of an anchorage, single handed, under sail alone is great fun.

One disadvantage I see in a marconi ketch is that the mizzen rigging can clog up the cockpit area. Now I am a fan of unstayed rigs, but surely there are some very good arguments for every mizzen to be unstayed.