You can activate SIRI only by pressing and holding home button your iPhone.
So if you iPhone inside the cradle, you will have to lift the center dash cover and press home button.
Or you can do the same via BT, but you need to have your phone in hand.
This limitation is from BMW and Apple, they just need to have integration between car and the SIRI (Mercedes announced such integration int he future)
For example, Pandora App is integratable with the car, same for twitter, facebook, iPhone calendar, Web radio (BMW apps option required)
1. BMW Connected app allows twitter, facebook, calendar, web radio integration
2. Pandora App is directly integrated with BMW not through BMW Connected app.
3. iPod App integrated with the car only.
Those 3 methods will only work when your iPhone connected via USB or in cradle.
On another hand with blue tooth integration you get contacts from your iPhone and audio streaming from your phone via BT (extra option for that)
Bluetooth audio streaming gives you less control over your music and options in iDrive compare when you connect via cradle or USB port.