That is what I thought, then I saw this video:
https://www.youtube.com/watch?v=pI3N_a7EydQ&feature=related
It seems to do the job. I think the shock value would give valuable seconds, enough time to escape pronto that is.
I think anything in the eyes is irritating, even opening them under water is quite hard to do.
I mean, if someone is in your face, or robbing you, it is better to have something than nothing and they would be stained for days after, making identification easy.