An app with the cards is a cool idea, since PDF load times on my phone take too long on each page.
An app that can replace physical cards during a game is a terrible idea, because I want to see, in front of me, all of my own and my opponent's cards, including counters, dry erase notes, and damage sustained. List building, game generation, references and auto-updated errata/FAQ are awesome, but possible shenanigans during a tournament is not cool. I do not believe in the honor system when prize support is on the line.