SPI- Full duplex and Half duplex
Half duplex unidirectional or write only mode could be used when master don't need data from slave. For example when working with most LCD it is usually OK to do writes only (although by reading status, framebuffer etc master can detect misbehavior and reset the slave, but LCD is usually not critical component and scheduled blind resets might be enough). The advantage is, as you mentioned, saving 1 pin.

Read/write clock rates are always same, since it is done simultaneously. MCUs SPI read and write buffers are shift registers with same clock, so when master is writing it is also reading (even if slave is not sending, in that case HIGH bits are received).