Linear bearings are fine, in the right application and if properly specified. My vertical mill uses cast-iron slides. My current router uses profile rails. My 3D printer uses linear bearings. The issue with the printer is not that it uses linear bearings, but that they use 8mm unsupported rods in a frame that does not provide adequate, stiff, support. Given the overall system accuracy needed for 3D printing, and the relatively low loads on the guide rails, I don't see a problem in principle; it's just that my current machine is too flimsy. In a commercial machine where you can machine mating surfaces, it might be quicker and easier to use profile rails. In a home-built machine where you build in adjustment and have the time to set up accurately, there might be a better way. Look at the system design and don't get hung up on point details. I shall be thinking about the overall frame design for my mk2 printer and choose an appropriate guide system in the context of that design.