It's unlikely that you'd be able to use a cellphone camera on the RPi. They're very tightly integrated into their motherboards, which makes getting them off the board difficult. If you manage to do that, getting drivers for it and connecting it to the RPi will be a nightmare all its own.
The RPi camera module was considered big deal for a number of factors.
CPU load. While there's a lot of benefit to the USB standard, it produces its own set of headaches. The USB standard is incredibly complicated and adds a ton of overhead to anything that uses it. This isn't a big deal on a normal desktop computer because they usually have a lot of resources to spare. On the RPi, this is much less
Remember, the USB ports and the ethernet all share the same bandwidth. If the camera starts using the majority of the bandwidth, your network will start to crawl and could adversely affect your video performance.
Most USB web cameras won't support hardware encoding at the cost of the RPi camera module. The RPi camera module is $25, whereas you'd probably need to spend $80 to get hardware encoding built into a USB device.
While it's true that the camera module itself doesn't provide the encoding, the DSI connector allows it to connect directly to the GPU, which does. Due to the nature of the SOC, most USB cameras simply won't be able to provide hardware acceleration unless they do it internally.
While connecting a USB device camera isn't necessarily awful, it certainly isn't as compact as the connector style of the RPi camera's ribbon. The form factor allows for a much more compact, and flush mount that you could get with any USB camera I've ever seen. The RPi camera is also significantly lighter and is supported by many popular cases.