I have to wonder if, in some cases, the "well, this person fills this role, while this person fills this other role" type of phrasing is really just trying to find a way of phrasing after the fact.
For example, Joey meets Cindy. They work in a technical field and share some common, "techie" interests. Joey also meets Rachel. They meet at an art gallery, and they share artistic interests.
Joey could say that Cindy fills needs that Rachel doesn't and vice versa, but really, it's more of the situation defining the "needs" rather than the needs determining the partners. He may not have been searching for a more artistically-inclined partner, but found one.
In other words, not sharing an interest of yours with a partner doesn't mean that you're actively looking to find someone who has that interest. However, when you find that person, it's like "Hey! Cool! I can share <x> with them!"
But then something gets lost in the translation when you try to explain it to people...
Mono. Divorced, two kids (DanceGirl, 14; and PokéGirl, 11), two cats, one house, many projects.
My partner. Poly. In relationships with me, Xena, and Noa.
Poly. In relationships with Chops and Noa, and dating others.
Married, Poly. In relationships with Chops and Xena (individually).
Blog thread: A Mono's Journey Into Poly-Land (or, "Aw hell, there's no road map?!")
Slightly more polished blog with a mono/poly focus: From Baltic to Boardwalk