I have to agree with the people who have stood behind RS232. It is very simple and effective for this task, interfacing with small microcontrollers.
Very curious things can be done if the EIA specs are ignored. For example, we were able to program the 2003 RC when connected to a PC through a daisy chain of cables over 150 feet long. Try that with USB.
Also in 2003, we came very close to the secret of having one Basic Stamp program another on the field, all because we were using RS232 connections between the RC processor and an auxiliary one. In fact, interfacing an auxiliary processor to the programming pins would have been completely impossible (or close enough to) if the port was USB-based.