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Most people in this thread object to prompts because they aren't aware of how often pros use them and they errantly believe that the professional way is to memorize everything. It's more accurate to suggest that high level shows have too much at stake to leave anything to chance; you are much more likely to have prompters on the floor in that setting than not.
The pros use them a lot, but they've also gotten good at doing it in a way that nobody really notices. It's not the prompts that are an issue, it's the delivery.
Agree that it's the delivery.
I've got this issue going on with my current band doing rock & dance covers. Our singer has a borderline enormous voice, but I did quite the

at first when I saw her using a tablet for lyrics. I thought that maybe these songs were new for her, but she uses the tablet all the time.
Oddly enough, since I got in with these guys three or so years ago, I've seen a couple other local bands (cover crews like ourselves) with a singer using a tablet, too. Some manage it better than others. I guess that I don't mind it so much with other bands as long as that singer has a knack for "delivering" the song, but I keep finding that I'd rather be playing in a band that isn't doing what can feel like a sort of karaoke.
If our singer is too focused on "reading" and not "singing", that's when it's a little cringe-worthy for me. Lyrics can get out of sync with where they belong in the music. That's where we achieve true hacker status.

Unless a singer has that ability to really deliver the song while using a tablet - sing it and not just regurgitate it - I think that it can bring some significant limitations to an act. It's an issue for us.
On top of this, our lead guitar guy has a very average musical ear... and his tone can sometimes get ugly. I guess that's another topic for another thread, but it doesn't help with my overall tolerance for this situation. I'm keeping an eye out for another possible band, but still going along with this one for now. Don't want to leave these guys high and dry, but I'm really not enjoying it anymore. I'm also dreading the possibility of getting booked to play a bigger show at a more established venue where I think we'd be underwhelming... at best.
@gotly : Maybe I'm a bit further down the road in a situation that's similar to yours. I'd say be diplomatic with your encouragement and recommendations, but offer thoughts/ideas without being confrontational. You may have to live with it, but it also might not be a problem.