This is my opinion, and musical elitists won't like it (enjoy the music for it's value, you aren't a real fan then, etc.) but if you go on tour to support an album, then play three songs off of the album, chances are people will be mad. Especially newer fans like myself.
I was upset, for sure--I went for the Neko sound. She is what I enjoy about their music, her sound in particular. She gives them an undeniable edge.
And I can appreciate good music, sure. But when I'm expecting one thing and get something else, I think I can justifiably be upset. No one wanted her to go on when sick, we're not a-holes. But if I don't show up to a meeting I'm supposed to lead, I usually cancel/reschedule.
Okkervil River was freaking awesome, though, and worth at least 3/4 the price of admission. They showed up NP IMO. So maybe, the cancellation would've been bad. I change my mind on that.