General agreement with the overlap being a big factor, and Nintendo consoles also do generally have a bit more of a local multiplayer focus, with plenty of games that are really at their best when you play on the couch with friends. So, in my case, buying a WiiU as a secondary console will largely be because, when there's gaming parties to be had, that console will be taking the spotlight, whilst the PS4 largely remains neglected past; "Oh, let me show you this new PS4 thing real quick before we start!" Price might also be a factor, as when it comes to spending another chunk of money on another platform, it helps lower the barrier to entry a little if said chunk is smaller than what you spent on your primary console.
The only real reason I could think of NOT getting a WiiU as a secondary is if one actively, unequivocally dislikes or is uninterested in the types of titles Nintendo excels in.
Zanten, Doer Of The Things
Unless He Forgets In Which Case Zanten, Forgetter Of The Things
Or He Procrascinates, In Which Case Zanten, Doer Of The Things Later
Or It Involves Moving Furniture, in Which Case Zanten, F*** You.