Are we talking about the Fusion strategy where both devices share the same library? That's what i have in mind to give my opinion.
Cons: less HW sold in the Nintendo fanbase.
Pros: Better, more diverse library helps it sell to the MS and Sony crowd.
Other Pros would include financial gains on game development budgets and no more marketing sacrífices (Wii U was sacrificed in favor of 3DS in 2013), amongst others.