I have taken the time to assess this issue and its patch, and I have come to the conclusion that I would not want this patch (as it is) in any of my implemented solutions, nor would I advice other implementers to implement it.
For the record I have tested this in a new 13.07 checkout against demo sales invoice 8009.
While the title of the issue and the description could answer the question 'why?' it seems to me that the use case has not been thought thru that well in relation to functions currently available. Nor does it seem that other requirements (that may be out there) have been taken into consideration. At least it is not mentioned.
The patch does what it is intended, namely take the details on an invoice and create a credit note of it. But it takes all, like payment date and applied details, but also the original details regarding VAT and other taxations. Details like these should not copied, but recalculated in stead. Payment of the credit note is likely to happen after the credit note is created, and VAT and other taxations may have been changed between data of invoice and date of credit note.
Thinking of it, the patch may well work in a situation where a purchase or sale not to/from stock but to expense, service and subscription) has taken place and the organisation doens'nt want its employees (users) to go through the motions of creating a return order and such.
But I would advice the community to not implement this patch in any development branch.