You have some options. You can do what has been done since the invention of the automobile and deal with kids in the car... A time honored tradition of siblings fighting, yelling, kicking, screaming, not-not touching and the like, with the occasional road game/song... Or, you could let them use this time, as they always would, by watching some in flight/ride entertainment.
Personally, I consider the latter an improvement over the former (from oodles of experience being that kid without, and then with, including the progression of Detroit-Daytona Beach drives before and after the invention of the affordable PMP) and I therefore choose the latter for my self and eventually my family...
Nobody is forcing you to do it, and if you think that there is little to no value in large capacity PMPs, you are under no obligation to buy or use them.
It is just an additional option in the marketplace, and I appreciate having the option...
Of course, I'd rather use a simple inexpensive laptop rather than a 'slate', but hey, that is just me... And the advantage of an inexpensive laptop is I can quickly swap out HHD/SSDs, so that I can have the best of what I need when I need it...
More options is 'more better' for consumers.
Also, by creating more drives with higher capacities, this puts more pressure on SSD manufacturers to keep pushing the capacity envelope.