Before making a commitment to have someone be an advisor, test drive the relationship first. See if you can work with that person for the longer term. See if you like the perspectives they bring, their network, their attitude, or some other intangible/tangible benefits. See if your other advisors value their contributions.
A great framework is to work with someone for 3 months, and then revisit the relationship and see if it's worth formalizing. Good advisors will be confident and welcome the opportunity to show their value.