For the record the belt tensioners are a very good idea. They REMOVE elasticity from the belt, not add it. As cheap belts age, they might stretch or the reinforcing elements can snap. The belt tensioner simply takes up any slack.
If you get a lot of stretch in a belt - particulatly on a delta where it's easy to see. A belt tensioner on one belt that is closed more than those on other belts, is a good indicator that the belt has stretched and might need changing.
Mainly they simply ensure that all belts remain at the same tension at all times.
Placed against manual tensioning systems - where people adjust based on the sound a belt makes when 'twanged' (honest, I did not make that up)- a spring loaded belt tensioner is like a digital thermometer versus someone sticking their finger in and saying: 'yeah that's warm enough.'

ok the linear rails thing is unclear.
I assumed they were used for the vertical movement of the carriages. With rods attached in the normal way to the effector and carriages. The idea was to take any movement out of the wheeled carriages.
So, if not that - then what ?