You can deploy a SPD workflow  in multiple locations for 2010; why the need
for copying?

Ok, how? I know I can create a 'reusable' workflow, but it would have to run on
the same content type. So I wouldn't be able to run that 'reusable' workflow  in
my Task list, Calendar and Announcements.

The XOML is still available unless your administrators have turned off
access to the All Files Node in the Site Objects list. Once you've created
and saved your workflow  if you go to the All Files list you will see the
hierarchical view that used to be the main view in 2007. From there you can
drill down until you see the .XOML file.

I believe the original writer wanted to copy  a WF so he could re-use it AFTER
some changes.

While I have not seen nor used SPD2010 & WFs, to me the most desired feature
would be a "WF snippet library" which used relative GUIDs rather than absolute
GUIDs. Does SPD2010 provide anything like this? I know it has "reuseable"
WHOLE WFs. Or can one "reuseable" WF call another "reuseable" WF so in effect
you sort of have a snippet library?

Interesting line of thought. I may have to investigate.

Reusable is indeed reusable. It calls other workflows, lists, all referneces
etc using Names and not Guids.