Re more pixels = more light, the area blocked by the grid would increase as a percentage so yes potentially that is valid (depending how far they shrink the transistors), as is the need for more transistors which could use more power.
Has some good pictures of TN screens which should illustrate the point.
As regards who needs more pixels. Need might not be the best word for it. As a photog I would love a 4k 17 inch IPS panel laptop, assuming programs can scale menus etc, leaving content to be displayed at the higher resolution. Long gone are the days of looking at a 6x7cm viewfinder or higher (large format is rarely digital, in medium format the closest is the hassie v system) and whilst the rear lcd's are getting better they are more use for histograms. Shooting tethered in the field can be really useful, especially when your depth of field is in mm and you can't reshoot. A shaded high res laptop screen is the dogs for verifying you got the shot.
Doing your post production with your feet up in the garden is pretty nice as well. Do I need a higher res screen? No. Would it be worth the money, offer some competitive advantage in a competitive industry and make life a but more fun, hell yeah :-) Obviously the same cannot be said for everyones situation. I would have thought dell or someone with a similar sales method would have put higher res screens in laptops as an option. The first one to do it would sell a lot.
If you can get a decent 1080p quadcore ivybridge with 8gb ram, a kepler gpu (for adobes mercury video engine) and an ssd for maybe 1400 usd, you could charge 600 extra for a higher res screen, undercut apple, sell plenty and make money.
Apple have moved first, if nobody else follows they get my money, plenty of others will be doing the same. Hell, a decent camera is 3-8k (you tend to buy 4 of them, 2 to use, 2 as spares), a really good one 40k, plus another 30-50k in lenses, and stills is cheap next to video, an extra few hundred on a laptop screen that will last maybe 3 years (that's pretty good next to a camera body which would have had its shutter replaced probably twice in that time), that's an easy choice. For a home user, it's probably a lot harder to justify to the mrs :-)