but it's not easier to bring a tablet and a laptop than to just bring a laptop, right?
No, but it is easier to bring just a laptop than a tablet. Any USB source charges it. You can carry it more easily, and use it in more places.
since a laptop does everything a tablet does
Not actually true (touch interface far better for things like drawing) but let's pretend it is.
A laptop may be able to do everything a tablet does but if the tablet is far lighter and has better batter life guess which most people would rather have on a trip?
In a foreign country would you rather wander around looking for a WiFi cafe with a tablet tucked in your purse, or a backpack with a laptop?
This is exactly why tablets have done as well as they have, because they replace laptops for some scenarios where portability is more convenient.
i'm in BT range of my phone 99% of the day, and i think that's pretty typical.
And I'm in quick viewing range of my phone 99% of my day, which is also typical. They are the same thing...