My boyfriend is my best friend and tends to be MUCH more mild mannered and accepting than I.
So I do frequently ask his advice in terms of issues between dh and I. Frankly-without his input, I would have destroyed this marriage a long time ago.
He's much more likely to support a solution that benefits my husband than anyone else in our lives AND he knows both of us well.
On the otherhand, if a lover didn't have a good knowledge of both of us or they weren't prone to supporting the relationship between DH and I-then I would definitely not go to them for advice.
I dont think primary/secondary or lover/nonlover matters. What matters is the persons ability to prioritize suggestions that are beneficial to both your partner and you. Some people are gifted in that-others are not.
"Love As Thou Wilt"