I personally feel that Digital Comics should not be treated any different Digital Music.
When I purchase an MP3 file, I can actually access the file, copy it, back it up and play around with it.
If I buy a comic off Comixology, there is no straightforward way I can access my file. I cannot back it up, I cannot view it on any other app. I am at the mercy of Comixology to read the comic.
That extra bit of control is what makes me apprehensive of buying comics...