Contrasting the older opinions, I find it refreshing that the world is larger than just the two of them. Few of us only have a single other person in our periphery, and unusually attractive people are often found attractive by more than one other person at a time. I don't think of it as an "intrusive rivals" situation, but rather a more realistic life for a person that is inconvenienced by the attraction of those around her.
Shimamura is surprisingly aroace-coded. I relate to that strongly, having been through a couple situations with friends getting irrational crushes on me and causing me distress or discomfort as a consequence.
Also, these moments with Tarumi lead to Shimamura becoming increasingly more aware of her own feelings through observing the comparisons and contrasts between her two suitors. She also makes us readers aware that Shimamura appreciates Adachi more than she herself recognizes, to the point other people can tell when Adachi comes to her mind. Tarumi is a foil, if anything.