Stuart wrote:

Could this system be used with pre made standard size carbon fibre tubes? A cheapish carbon fibre mast :)

 Yes, if such tubes can be sourced at sizes, prices and specifications that make them suitable for assembling into unstayed masts, then 3D printed components can be used instead of having to hand-make joining sleeves, which tends to be a tedious and messy business. However, I’m not aware of any such tubes being available. There are CFRP and GRP tubes in sizes suitable for making battens and yards, though, I tend to prefer them to aluminium tubes, and 3D printed additional components are a convenient alternative to other means of manufacture.

In essence, a 3D printer can replace the lathe, mill, drill etc needed for machining components from solid stock, or the welding and bending kit etc needed for metal work, with only a very reasonable up-front financial investment and a less steep learning curve than for machining and welding.