It is related to the svg
structure.
Gradients, pattern fills, custom markers, clipping paths are all stored in
that part of the file. If you set it on an object, the information is linked. If you use copy/pasting, then the two objects will have two different data for their gradient/pattern fill/...
In my experience with 0.48 it was the most common reason for program crashes, that too many defs were stored.
Causes serious lagging at least.
Mostly related to a bug with the gradient editor, you could end up with thousands of unused gradients fast.
To manage this, there is an option in the file menu to cleanup document (previously vacuum defs), that deletes unused definitions.
Well, other than duplicating, some cases you can use cloning (Alt+D) -if you want one kind of object repeated, and edit all the objects later on the same way.