Part of the answer to your question about why OKE is a corporation and OKS an MLP--and the reason I own both--is that there are restrictions on IRAs and other tax-exempt entities owning MLP's. I own OKS in my taxable account and OKE in my IRA. Both have dones very well.
Also, OKE was a regulated public utility long before MLPs came along; unwinding that structure might not be easy or cheap. Scott