The "value" is that it won't cause your 802.11g network to crap itself when it sees an 802.11b node. Thus, your N800 won't cause other devices on the same network (e.g., your laptop) to suffer a network performance hit when it attaches to the access point.

(When an 802.11b node associates with an access point, the access point enables various features to ensure the b and g nodes can interoperate, such as CTS/RTS for every frame. So your g nodes take a huge performance hit the moment a single b node associates.)

Also, as I believe someone else already mentioned, in g mode, the radio takes less time to send a frame, therefor the device uses less overall power since the time spent powering the transmit antenna is reduced. This extends your battery life. (At least for devices like the N800 that never have to worry about sustaining full bandwidth wireless transfers...)