It has bash/permalag immunity even outside going into a shell by the thick carapace that bounces crap off it like its nothing. The only time armadillo is a pointless form is when you already have tortoise as a third tier. They are copies of one another with armadillo having slightly better ratings.
I rank armadillo easily at #3 or #4 of defensive forms at final tier. I put crocodile and porcupine and komodo dragon above it. With the rest under it. Its still absolutely an awesome defensive form, especially for getting out of really nasty situations where you might need to bounce 30 flees before you hit the right exit through hordes of nasties.