First, DPCCH and DPDCH are time-multiplexed in the downlink direction.
Note that if code multiplexing was used, each DPCH (DPCCH and DPDCH) requires two channelization codes, and is less efficient.
Second, the control channel must always be present because of the power control mechanism. But, what if there is no user data?
This will result in a pulsed transmission, which is not a problem in the downlink, as common channels (CPICH etc) will be sent all the time.
In the uplink, however, the discontinous transmission with time multiplexing would cause severe electromagnetic problems.
