Unlike the in-seat entertainment, the movie on a personal tablet won't be interrupted by announcements which I almost never want to hear.
If you want to spend most of your time reading, a Kindle may have advantages but I'm more inclined to travel with a tablet. The newer magnetic keyboards are actually pretty good but I'm at fewer conferences and the like these days where I'm taking notes and I actually have found that I've adapted to just using a phone for many purposes.
You actually give me an idea that I have an old iPad and a laptop stand I never use (probably a work thing I didn't think to return) and I may give it a try especially given I mostly use just a few sites.
What I think I would really like is a detachable keyboard with laptop-style hinges (and maybe a secondary battery). There seem to be a few devices from about a decade ago with that kind of design but they died out in favor of flexible connectors and kickstands.
A more hybrid design seems to make a lot of sense but it just seems to involve compromises on both the laptop and tablet ends. Even the Apple tablets with keyboards probably don't have a lot of advantages over the Air unless you're watching a movie or reading a book on a flight or train.
It would still be pretty compromised as a laptop, but better in that role than current tablet keyboard solutions.