You can use a single VCO in systems where you transmit and receive in the same box, and can share the VCO between them. A short range FMCW radar is an example--the vco can not drift very much between transmit and recieve times, so you do not have to lock it. Toys, cheap radios, etc can also use unlocked VCOs.
In applications where you need to receive at a distance what is transmitted, and you do not want to tune the receiver LO vco to try to find the signal, will need a locked VCO.