I’m not sure abt any situation where we need to disable DTX. But we may like to disable power control when we have severe interference issue. Lets say in an area you have very high external interference due to a jammer or something else and interference level suddenly increases. In this case you will like to transmit at maximum power always to achieve best possible C/I. Of course PC algorithm takes into account the quality but increasing the power in poor quality conditions still takes some time during which call can be dropped.