I think the rationale is that by separating the faster growing midstream assets into a separate MLP, this will provide more visibility and the market will assign a higher valuation. Since NRGy will retain an ownership interest and the incentive distribution rights, they will benefit.
It isn't being spun off to NRGY unitholders. NRGY is going to IPO a new MLP that will hold the Marcellus midstream assets. NRGY will continue to hold around 70% of the entity. It will bring in $200 million to NRGY and will give the midstream assets a low cost of capital platform.
I personally would rather see them divest the propane operations like Energy Transfer but they seem interested in maintaining their propane business.