Being able to swap out the phone dialer with a SIP client means my Droid can make and receive calls when I'm on Wifi in Europe, where it's CDMA network doesn't exist.
More importantly, this is a replacement done by the user. Being able to customize your own OS is always (in principle) a benefit. I'm talking about things like AT&T hiding Google Navigation on some of its Android phones to encourage the use of its own paid navigation app.