A while ago it was decided to split each instrument into its own relationship (but merge them for display in the UI). This was done to make data entry and storage more consistent; previously an editor could* set any number of instruments on a relationship, which meant that all of the following were allowed to exist simultaneously:
original member: Ian McDonald (background vocals)
original member: Ian McDonald (background vocals, flute)
original member: Ian McDonald (flute)
original member: Ian McDonald (flute, saxophone, mellotron, keyboard)
Note how each relationship could have a different number of instruments, and could overlap/duplicate instruments on other relationships (so long as the sets weren’t identical). Splitting them was a way to standardize how they were stored and prevent such duplication.
(*It may still be possible to edit an existing relationship to have multiple instruments.)