pig-
Like supersix mentions, versatility is one of the reasons. I enjoy science and engineering, so a multidiscipline field like Materials really appeals to my inner geek. I'm interested in materials research as a whole, in nanotech-related fields and also in the non-CAD concepts behind microprocessor design. Since ASU has some good nanotech research labs, I'd be able to get a lot of hands on.
Part of the versatility that keeps me interested is being able to move from field to field as needed or as opportunities arise. If I stuck with ChemEng or MechEng, then, while still broad, are slightly more limited than Materials AFAIK. Also, I do believe I'd like to stay in academia, doing research and eventually teaching.
I guess it comes down to the fact that I can think of some reasons I'd like to do it and no reasons that I would not. School doesn't scare me!
I've been warned from several people that the program will be hard, hard, hard. But science and math are, while something I'm a bit out of practice with, my favorite subjects.