Less customisation. Right now, your fresh install is broken out of the box. You set up ubuntu and sound works. Plug in a usb phone and configure skype to use /dev/dsp1. Cool. The next time you boot the computer, all your system sounds are coming through the phone and skype is using the system speakers. Most users want devices to NOT MOVE ABOUT.
Does this actually happen? What I've seen is Linux assigns things based on something like PCI order, which is likely not to change unless you move cards around. With USB things, its possible that plugging them in in a different order could swap the /dev/dsp numbers, but with a single PCI sound card and a USB one, I wouldn't expect it to swap. I only get network interfaces changing on me between kernel/udev updates, if at all - never on just a reboot. Craig