Most devices supported by OpenWrt have or can be modified to have a serial port. However, some bootloaders don't have a “really failsafe” network re-flash feature, making a serial console one of the few ways to recover from a “bad flash” or an error in a user's own configuration. Most people get along without a serial console for their device because they're able to flash a working firmware the first time - or are able to apply various recovery methods - and do all their communicating with the device over a network.
