Indeed, Johnson had many positive things to say about his new friend, calling him "quick-witted", able to "make light of any situation" and someone exceedingly "proud to wear the uniform of the Union Army".
Still, it was not Johnson's idea to bring Smith into his command. Instead, the minor act of US Army racial integration was largely due to the influence of the Fifth Doctor. He had suggested to Johnson that Smith would make a valuable addition to their expedition to Richmond, because Moses, being a native Virginian, was familiar with the back-country around the Confederate capitol. In truth, though, his most valuable asset — as far as the Doctor was concerned — was that he knew the slave railroad in the area, and the people that comprised that railroad would definitely have noticed Peri and Erimem travelling overground.
In a subsequent confrontation with the slave-catcher, Jubal Eustace, Smith proved most useful in applying military pressure just when the Doctor was confronting the colonel's hardened racism. Smith was the also the person who brought Aaron Eddowes into custody after the latter's attempted assassination of Abraham Lincoln. (PROSE: Blood and Hope)