I don't think you are over-reacting, but I do think this rates somewhat lower on the scale of things to be ticked-off over.
Highest on my list is unexplained no-show - you wait for someone who doesn't show, doesn't call, doesn't e-mail, and doesn't answer the phone - that's the worst and least forgivable.
Next on the list is the last-minute cancellation - flat tyre, a sudden bout of bubonic plague, whatever - sure, it might be real, or it might be that she just doesn't feel like getting out of bed. Had one recently where the model had decided to stop modelling, but her reason was a good one (I won't repeat it, because it was personal), and I felt compelled to forgive her (it helped that my first call to a replacement model was successful, and I got some great shots from the replacement shoot
Next on the list is the flurry of last minute changes - she wants to shoot 20 minutes later, she'll be another 10 minutes, she shows up half an hour later than that. Hmm, this might almost rate number 2 on the list.
So I'd say that being contacted well in advance to cancel or postpone is probably fourth (maybe even fifth) down the list, and I'd be less annoyed about that - still gives me time to find a replacement model.
I have had all of these in the last six monthes, and this is the order of diminishing ticked-off-ness for me.
I don't know if that helps, but maybe it does?