It's not a shortcoming of RT, is just that the hardware atm is not strong enough, so fewer rays than necessarily are being cast and the AI just "fills" the empty spots. Grain I think is the result of that. I haven't saw that grain in Metro or in the little bit of time that I've had RT on in Tomb Raider, so it will depend per implementation.
Path tracing implementations currently are quite noisy due to having to sample over several frames as well as not having the hardware for higher ray counts so would be a little amusing if the grain was due to the AI trying to fill in the blanks with noise due to seeing the need to reproduce more of the same as the ray tracing output heh.