Probably not. Most ebooks "floating out there" are already in epub format. If anything, this makes it easier because they don't have to be converted to mobi.
As has been said lots of books are already out there as epub, there are many that are already in Amazon's file format too. I'd guess that most people torrenting books are using calibre anyway.
Why would someone do this? Isn't it basically asking to get the account deactivated due to piracy concerns esp given that now Amazon has the whole trail of things you have pirated?
People do it because documents that go through the e-mail thing sync across devices like Amazon purchased books.