Ah, of course!

I have sung from an iPhone on one occasion. It was in the bar of the Half Moon in Oxford and someone asked for 'Lady of Autumn', which I'd never quite learned properly. Nipped outside to grab a signal and the lyrics, paced up and down the street singing it through in double time, then back into the bar and went for it. Only had to refer to the crib once.

I'll only use it as an aide memoire for songs I half know and only if there's a request and nobody else knows it. Ideally only if I've got time to do the pacing up and down outside running through the song a couple of times thing beforehand too.