Diana Krall is used for demos because "Live in Paris" in particular is a very good recording with good music well played. It generally makes equipment sound good and Krall's voice is warm and expressive. Voices are good for demos because we easily relate to them.
What turns some music lovers off Diana Krall is that she isn't that good at digging down into the heart of the lyrics - many singers do it better from Billie Holiday to Ray Charles and many others since. The thing about jazz vocalists is that it isn't just the lyrics that matter, it's the improvisation around the melody, which Ella Fitzgerald was so good at. So the more they sing "like a sax or trumpet would solo" the less the lyrics may matter. If you want the most feeling out of lyrics, you're more likely get it with popular singers, soul and blues singers and singer-songwriters.
So ultimately Diana Krall doesn't quite satisfy. But she's a very good pianist and a total musician. Ginger Rogers use to quip that everything Fred Astaire did she did backwards in heels. And everything Krall sings she does while playing excellent jazz piano. She's not the only female singer-pianist though - long list here including Carol Welsman, Diana Schuur, Blossom Dearie, Nina Simone, Kandace Springs, Norah Jones, Eliane Elias, Tania Maria, Laufey, Aziza Mustafa Zadeh, Nilüfer Verdi, Patrice Rushen, Shirley Horn, Patricia Barber, Clara Ponty.....