why does stevenson use utterson as his point-of-view character?

Utterson needed to be a reliable narrator. For this reason Stevenson described him as “measured, and at all times emotionless, bachelor—who nonetheless seems believable, trustworthy, tolerant of the faults of others, and indeed genuinely likeable." So, you are right, he does tell both sides of the story. He is, however, not completely uninteresting. He does show emotion like guilt. In the end Utterson proves to be a relatively unbiased guide into this very strange tale.