Danielle Woerner wrote an interesting post about memorization as a singer
(among other things) that's too long to quote really.

I'm wondering about language/textual problems, that are unique to singers
(vs instrumentalists).  It seems to me that singing a song in say, Czech
 [assuming you don't know Czech] would be more difficult without the music
even if you had every note memorized.  Is this a factor when singers decide
whether to use the music in performance?

Bob K.