If you've got a bigger plane, you also need a bigger runway. This thing should increase the number of usable airfields. That could be interesting for avoiding more expensive routes.
It is, but that only happens at one end of the trip. It can still drop cargo on a smaller airfield at the other because the braking can be done independently by both.
Plus there's an interesting wrinkle here: a lot of the media on the site show the tow plane as one with engines, and not a glider. I'm sure that's because those are the aircraft they could get their hands on, so that's what they made it work with, but the option is there to have the towed plane powered on takeoff. Doing that safely, with the two aircraft tethered... that sounds like a mess. But it's an option!
Your towing plane is going to have to accelerate the same mass to takeoff speed before it can get off the ground, no?