I think it's a fad to sell more TV's.
There used to be a time when the average US household bought a television once every 10 years, maybe longer. And the manufacturers found a way to survive on that.
Then HD rolled around and the companies loved seeing the profits from new TV's that they decided they wanted to see those profits every few years. I believe they embarked on a campaign to make it seem like even for HDTV's, that you needed to get a new replacement every three or so years. To do that you need some kind of advancement.
First you had 1080i TV's and 720p TV's, and even 480p "EDTV's" targeted for a different market that didn't want to spend the big bucks for HD.
So then came 1080p TV's, which everyone had to have. Then came LED backlit LCD's ("oooh, sooo slim!!"). Now 3-D is coming along, I believe all to make sure you buy a TV every 3 years instead of every 10-15 years.
Ask any AVSer how many times they have bought a new HDTV for their primary viewing space in the last 10 years. I bet it's on the order of 3-4. For me it's three from 2005-2010 (for my living room, the primary viewing space for me). The manufacturers just want to get us again...and again...