Well, it's not that simple...
It's not that the G routers drop to B-only mode as soon as a B client connects, it's that B and G use totally different radio transmission methods. When only one type of client is connected, it's simple: the router uses the appropriate radio mode. When both clients are in use, it must switch back and forth between modes, with each switch taking up time. In the worst-case scenario, it must switch so often that more time is spent switching than transmitting, so the overall throughput drops down to B levels.
In practice, it's not a huge problem if it's just two or three computers. Most of the time, they won't be transmitting much, so the slowdown is negligible.
tooki