AS Bernard says, some people, myself included, learn songs more easily by singing them over and over again, rather than sitting down with the text of the lyrics, so for me it has to be the car Cd player.
I have used the same method for learning lines from plays, which involves having to record yourself reading ALL the characters and then leaving gaps for your own part!
I would prefer to sing MORE songs, if using the words for some of them, rather than restrict myself only to those I can definitely do from memory: hopefully the former eventually become the latter.
And I prefer the term "comfort zone" for performing within a discreet sight of the words, available if needed, rather than a "crutch" - whether the words are in a neat folder on a music stand or on a crumpled piece of paper in your pocket or behind your back: it removes a lot of the anxiety about forgetting the words........which fear makes you almost certainly forget the words!