I agree that this gen will likely have even more people who only own one system. A lot of factors contribute to this and when you think about it, is there really a need to own more than 1 console? Especially, the PS4 & XB1 as they will share 95% of their games.
As of now I only have a 3DS and haven't preordered either new console. I do have an iPad (using it to post) and have to say it's an excellent device. It has tons of features that are both useful and fun. When it comes to gaming specifically I don't use it too much though but it still takes away my time from gaming systems. I play multiplayer games on it and it is a good way to kill time.
So I own an iPad for just about everything including simple multiplayer games, a 3DS for more advanced Nintendo games, and might buy a console down the line for multiplayer at home when people come over. That system will likely be a family system so I won't really get to choose which one myself and so far, PS4 seems to be the family choice.
For single player games, I like HH systems much better because they don't get in anybody's way when you play. You don't take the T.V, you don't have the sound blasting at night, you don't have people bothering you because they want to play too. It is just better. Funny thing is, I only play my 3DS at home. When I'm riding in a car, or in public, I normally just listen to music instead.
Short Answer: I will own 1 phone, 1 tablet, 1 HH gaming system, and likely 1 console. (Last gen I/my family owned all 3 consoles and both HH systems.