I can add to this since I often copy and paste definitions too. I do it in two circumstances. The first is what was already described, moving the definition I'm learning to the front of the list (most often this is just shuffling the parts of speech around, e.g. I'm learning a word as an adverb that means something else as a noun.)
The second is to edit the dictionary entries for characters that have a mix of standalone meanings and bound-form meanings. The ABC dictionary includes the pinyin of the two-character words in its (BF) entries, which means if I'm quizzing myself on a standalone character's pronunciation based on the definition, I need to get rid of that part of the definition so as not to give away the answer to myself.
For characters that have no standalone meanings, it's usually not worth the effort to edit the existing dictionary entries, especially since, in my experience, the ABC dictionary very often doesn't even list the compound I'm learning for a given character. Or it lists the compound but with a different definition than the one in the main entry for the compound. (The BF-only dictionary entries are, in my opinion, the weakest aspect of the ABC dictionary by far.)
Actually, a third reason I copy and paste: On my Treo there's no other way to input certain characters, e.g. a tilde or (more frustrating) a semicolon. I also sometimes copy and paste an entry and throw away its actual contents just so my own entry can have the nice-looking (1) (2) (3) markers, the diamonds between parts of speech, etc.