Hello James,
what confuses me most is that even when the currently running istance of Spaceclaim has no open documents, double-clicking any SC document on the desktop opens a new instance. I'd expect that at least then, the file would open in a tag in the open application.
Drag and drop has it's limitations. It is nice when you have room on your screen, but if you have all your files on a server and need to open tons of windows to navigate there, it is most unlikely to have any SC window open at the same time. Double-clicking the file icon just feels natural, then.
If it was configurable, I'd most probalbly set it to open in a new tag when double clicked. Good idea of yours.
Martin