Look at the huge pulley you have on the spindle, wonderful leverage on the tool, nicely counterbalanced by the motor and no gears to wear out or shed teeth at the wrong moment.
Not quite enough X axis travel? Swing the head off to one side and don't worry about repositioning the job out of square.
Loading a big lump of iron but worried about that expensive cutter in the collet? Swing the whole thing out to the side, safe.
Square columns are not necessarily square. If not square they bend differently in different directions. Round is round which ever way you look at it.
Want to drill in the top of something 4 feet tall? Stand it on the floor behind the mill and swing the head back over it.
Drawback #1 is when you switch from drilling chuck to collet chuck and have to drop down on the column. I usually center drill with the chuck, put the center drill in a collet, drop the head, put the centre drill back in it's hole. Tighten the head, done.
Drawback #2 is you can't locate the tool to the bed using fixed limit switches. I seem to manage okay without that.
Drawback #3 I'm sure there is a number 3 but I can't think of it. I expect someone will explain fairly soon
Accuracy? I can cut iron to better than .0005" on a good day with my round column but that took a lot of work. Mind you, same work it would have taken with a square column.
The later round column mills had squareness problems in the saddle, hope you have an earlier one.