I have used 80/20, Bosch, ITEM, Microrax, and probably a few others over the years and I have loved them and hated them at the same time. They made building and prototyping really easy, but all of them follow the Razer + Blades business model of selling the profile for a reasonable price, but all of the hardware and accessories are expensive. Since you are already in their system you are stuck. This problem is one of the 2 major driving factors in the design of our system.
standard hardware decreases the cost of every attachment point from $.40 per nut (.20 if you are lucky) down to .02 by using standard hardware. This gives you the build experience that is so awesome at a much more cost effective price for schools and teams.
Linear motion is normally hard or expensive. Over the past 13 seasons I have been involved in FIRST there have been many amazing products which have changed the game (things like the shifting transmission, systems of gearboxes, sprockets, and wheels that just work together). The one thing that has remained hard for teams is linear motion. There are low cost options like drawer slides and more pricey solutions like ground rods and linear bearings but up to this point nothing that was designed with building a robotics elevator in mind. The V groove bearings and integrated features in the 1 inch profile allow for super easy integration anywhere in your mechanism.
Oh I forgot to mention this on Facebook, but you can also use a NYLOCK nut in our channel for added security.
As for the corners there are a few reasons (some more obvious than others).
When you design a part to be aluminum extruded it is in your best interest to keep a constant wall thickness through your part, this helps with the flow of the aluminum (similar things come into play when designing plastic parts also). Secondly the amount of closed profiles in your extrusion makes the dies more expensive and harder to get consistent results. Specifically speaking to the 15 mm profile it would have been almost impossible to do a closed hole in the corner.
As mentioned above all 5 of the holes can be tapped with allows for some interesting applications, one of which is mounting a hub to the end and driving it with a motor to turn our extrusions into a shaft that could be used for a pickup. Along that same lines the slot in the corner is specifically sized to accept a 1/6" piece of flat stock (lexan or other), I am sure you can probably think of a few times this would be useful