Improvements in 3000m do not necessarily correspond to improvements in 1500m. If he is now spectacular at 3000, but has lost some speed side capacity in the process, say 1sec slower on his flying 200m, he will run the same or slower at 1500m. It is always a balance of speed and stamina which determines his 1500m.
Also, the distance work needs to be useful. Lomong running a great 50 miler would bode rather poorly for his track racing.