Thanks for implementing this, but the drag ‘n’ drop version is a little unsatisfactory. Shouldn’t “move” be the default mode, with “copy” requiring holding down a modifier key? And if there must be a modifier key, shouldn’t it be option rather than command? As it is, I can only drag ‘n’ drop one file at a time, because holding down the command key kills any multiple selection you might have made.
Hate to be a complainer, but…
Also, when do we get tags??
Just select the files, start to drag and then (!) press the Command modifier key.
I agree to acoyne. The other way round would be better …
But this is how it works in the Finder and we don’t like to disagree with the Apple Human Interface Guidelines. Before you know it their enforcers are all over us. 
That depends on wether you look at a database as different volumes or folders. Dragging in the Finder from one folder to an other will move not copy (if they both are on the same volume). And the modifier opt will copy instead of move.
As it depends on how you classify a DTP database I would say: both options are within the Apples Guidelines 
Therefor I would vote like acoyne and darwin to change the behavior. I can’t think of any circumstance in my workflow where I want to copy to an other database. If I’d want the same file in two databases I would not copy the file but index it within both databases. So 100% of my use is moving (and most moving around is from the Global Inbox anyway where copy makes no sense at all).
Johannes
I do it the same way like Johannes, never copy, ever drag from the Gobal inbox to the databases.
I’d also prefer the inter-database document dragging default be to move rather than copy, although one reason for it be copy is that, unlike Finder, there’s no way to undo a move.
As long as we’re voting, I’d prefer moving to be the default option too.