But this also has as a consequence that I cannot use it as a standalone device, it must be fed by my laptop. This is a crucial drawback. If I just want to stream from my cellphone, what do I do? Sucking out the power from my phone (if even possible) ?
It depends on the specific design and whether it will run without a USB data connection at all, but there are plenty of USB power adapters and even battery packs if you really wanted to use it on the go. Some phones will indeed power devices like this, too.
For low voltage / wattage devices USB power is becoming common even when it's not used for data. For example my GoPro uses a USB-powered battery charger, but it doesn't do anything with the USB except power.
Just look at USB as just another small DC power connector and you may feel much better about the whole thing.