There are two advantages to buying the carpet from ECS. The first reason is that the carpet is shipped flat. Since it isn't rolled up, there are no fitment issues. The carpet will fit the car like it's supposed to. The second advantage is that ECS orders NEW carpet. All of the carpet you buy is NEW. If you order it from another company, the carpet could have been rolled up, sitting on a shelf for years.
One other thing that the Moparts thread touched on is the fact that the 80/20 loop carpet has a tendancy to turn purple. I think this still would be an issue even with the ECS carpet. I don't know for sure...but that's what was said on the other thread. I believe the newer Challengers had cut pile carpet anyway so for owners of those cars it shouldn't be an issue.