Karma Support + Ardent Sensor....Why is no one doing this?

Sivir ult seems to be all the rage recently. Why was your time on a utility ADC for her ult when you can grab Karma support with Ardent Sensor.

Karma ult has super low CD. You use her ult with her shield spell coupled with Ardent Sensor and you just granted your whole team a shield, speed boost, attack speed boost, and 30 magic damage on auto attacks. That is a buffed up sivir ult every what 30 seconds? 40 seconds? Even if you still wanna pick sivir that gives you two Sivir ults in a row.

Haven't had time to play with this much but it seems like it has a lot of potential.