Just one more intrusion into your personal life by your employer. The last thing you need is to open your smartphone and see yet more company influence in your face on your time off. Reminding you that they own you.
I don't see why anyone would be excited about this. The time, money and effort could have been better spent on producing a decent rostering system in the first place.
I agree with the IT security point. I don't trust them further than I could throw them and won't be putting their app on my iPhone.