10) Brainiac is the biggest name Superman villain who has yet to be in a movie.
9) David Tennant has experience playing an alien.
8) Much like the Doctor, Brainiac regenerates into different bodies and is sometimes identified by number.
7) David Tennant has experience playing sciency-futuristic-type characters.
6) How awesome would David Tennant look in a big budget CGI movie AND Brainiac for that matter?!
5) Brainiac should speak with some otherworldly accent (perhaps English or Scottish, as long as it's not American).
4) David Tennant's certainly got the acting chops, and playing Brainiac would be a challenge for him.
3) Odd enough as it sounds, Tennant's body is kind of build like you'd expect a hyper-intelligent but still threatening alien nerd to look like...and his facial features too!
2) Brings in more British fans to Superman, always a plus.
1) Heck, it brings in more Doctor Who and Harry Potter fans as well!