Should you EVER use the DROP COLUMN command?

I do not agree with you. There are reasons where you really need to drop the unused columns e.g. when you are using the EXCHANGE PARTITION feature - which won't work as long there are columns set to unused.
And I think it's really a gap in the product that the MOVE TABLE does not remove the unused colums. I do not know any "online" operation to get rid of these unused columns. And as you stated, doing it "offline" takes a while, esp. if you have a large table with hundrets of million records.