If someone else (like a BE) is mixing, you can probably leave the ipad in the truck.
I wish I could release the email I got after an artist we work with regularly appeared at an event we did not provide for. The BE advanced the show with the firm and was assured that the relatively simple rider would be met. When he arrived, it was either mix side-stage or on an ipad. He did neither. He made them find a snake and move the StudioLive to an appropriate FOH position. The artist's manager sent a 4 page email to the festival organizers after the event, suggesting that should such be encountered in the future he would accept the artist fee and then personally go on stage to explain to the audience that the show was cancelled because neither the festival or their production company can read simple, 8th grade English, or alternately are incapable of telling the truth.
We're not talking about Journey or some big, international act, but a regional singer/songwriter & band with 4 mixes, 18 inputs, and nothing out of the ordinary. Their only "demand" was to have a real console in a traditional FOH mix position.
As much as I like the iPad app, when mixing a band I am not familiar with or mixing a multiple band event, I run a snake and mix the "old fashioned" way! It is unfair to expect a visiting BE to use an iPad, unless he request it.