[sneak]

Corporations can't have custody of children.

Corporations can't issue medical directives for an incapacitated person.

Corporate shareholders don't get hospital visitation rights or medical insurance via someone's job, et c.

[honestcoyote]

Very true. There's a lot that legal marriage provides which can't be found in a corporation. The idea is that incorporating would provide some benefits and protections that the poly family wouldn't have otherwise.