Good article, thanks. I see a sixth reason: you can lend an ebook to a friend, as you would with a a paper book, or enable a member of your family to read it on their own ereader. 
Amazon had devised a system to allow readers to do that, not sure it's still in place, but it was of kafkaian complexity... and only available in the US, far as I recall.

Naturally, deDRMIing ebooks should in no case lead to massive illegal dissemination.