I use it all the time, great stuff but you see it very little. Why?? Not sure but I assume it has something to do with the price. While it is not expensive (usually around 3-5 dollars a foot installed) it is extra money, and from what I have seen in the trailer business, most folks come in wanting the cheapest trailer they can get for the amount of money they have to spend. Just my take on it.
I started using it about 10 years ago, and would not order another trailer without it. It is probally the most useful option they sell. You can not pre-plan for every trailering experience, and the E-track allows you to be ready for anything. I like running 3 line of the E-track down the walls, one at about 18" off the floor, one around 36-40" off the flor, and the other one up by the ceiling, say 18" to 24" down. This pretty much allows you the flexibility to add a upper shelf with the various E-track fittings avalable, keeping items off the floor and out of danger. While the other two rows can handle any moving situation.
You can add the E-track yourself, but it doesn't look very nice and isn't nearly as strong as if you would have it installed by the trailer manufacturer. There, they weld it into the walls, and hide all but the actual pieces that are used to tie down. The welding makes the E-track much stronger, and will strengthen the trailer as well, by tying all the wall supports together.