I think that the drum sander starts to make sense if you are trying to flatten wider panels than an upgraded planer could handle. But you might find a bigger planer with shelix type head faster, easier to power and collect chips from, and more versatile.
Of course, I would never run a finished door through the planer. The sander is great for flushing up joints since the conveyor is soft enough to distribute unevenness on the bottom of the workpiece. But to avoid chatter, I found that I had to run very slowly.
For me, the amount of ROS sanding is about the same for a panel out of the planer vs one from the drum sander. I've even run end grain cutting boards through the planer with shelix head. If you go for a sander, avoid the cantilever designs unless all you run are strips that won't lever it up on the open end.
My 37" drum sander was a great stepping stone from a consumer grade sander to the industrial wide belt that is on a truck headed my way right now. (It is the same color as tod's. I can't wait!)