According to this website:
Although GSM operates in duplex (separate frequencies for transmit and receive), the mobile station does not transmit and receive at the same time. A switch is used to toggle the antenna between the transmitter and receiver.
What, then, is the advantage of using separate channels for two-way communication? Communications can never go both ways at the same time; why have them separated (broadcast channels aside)?
One reason is to help with radio planning - if powerful cell antenna's (i.e. the base station antenna's) broadcast on the same frequency they received on, then they would have a hard time 'hearing' the relatively weak broadcasts from the phones, over the relatively strong signals from neighboring cell antennas.