You're pressuming the only cost is the price of the iPad. What about the luggage space each one takes up? What about the extra weight you have to carry? What about the mental processing of trying to remember what book/film/app/etc. is on which iPad? Why take 10 iPads on holidays when you can just take one?

But there is a cost -- the characters would often have trouble carrying so many pads (or that many pads + a cup of coffee, thanks Janeway).

Each padd is just as capable, people can just choose to use more than one at a time.