John T., you say "always get a combo..." When do you use the belt vs the disc?
Greg being you asked I will give my full opinion.
First let me say I do more woodworking other than pen turning and cutting blanks and in fact that is not even 1% of my shop work. If you did you will see right away the advantages of having both because they do sand yes but it is the way they sand that can be advantages. When buying one or the other or both just makes more sense to have both in that case you need both. I could not begin to list all the cases that both come into play for it would be too long of a list.
With a disc sander the safe area to sand with is the center to left of disc. This is the area where the material is pushed down toward the table and makes it easy to control. From center to right it pulls the material up away from the table. Also the closer to center of a disc the speed of sanding is slower, less burning and less material removed. The more you get to the outer edge the faster the speed and more material removed.
With the belt portion the speed is always the same across the entire belt. The material is always being pushed down toward the table. It is longer so that an edge of a board can be sanded evenly. With the bench top belt sanders the end of the belt is open and I use that all the time on curved piece instead of having to go to a spindle sander (which I have that too)
With a combo to take it back to pen turning you can always put a course belt on the belt portion and a finer disc on the disc portion and switch back and forth. I never sand the ends of blanks on a disc sander, I always use the lathe.
The reason that they sell larger discs is because of what I said about the useful side of the disc. The larger disc has more useful side to it from center to left. I use to only have a table top but quickly found out a floor model with a 12 " disc is the way to go. I still use both.
Hope it sheds some light and remember this is only my opinion but I have been woodworking for over 35 years. Just sanding edges of blanks and taking corners off blanks can be done with either or sanders.