I think the attraction of a graphic novel for students is that they can read it in one day. This works for both reluctant readers (GNs are not as intimidating), as well as for students who love stories: They want to read as many as possible, as quickly as possible.
Also, some stories are better told in a graphic fashion. In the best GNs, the pictures are the poetry which convey the legendary 1000 words.
I'm trying to stock my library with as many alternative reads for students as possible. A great resource for buying and chatting about graphic novels is The Beguiling out of Toronto.
***Please preview these titles. Some may not be appropriate for your students.***