> What about cuben rain gear?
Well, if we ignore Gore's utterly fatuous claim of 'guaranteed to keep you dry', then there is considerable concensus that in bad weather you are very likely to get a bit damp inside your gear. One of the biggest reasons for this is that when your sweat hits the rain gear, it condenses on the inside. No, Virginia, breathable rainwear simply does not exist.
OK, given that you are going to get wet in sustained bad weather, will you notice a few extra leaks coming through the Cuben rain gear? Unlikely, imho. As long as it sheds 99% of the rain you are fine.
Let us move on to tarps etc. The same problem obtains: you get condensation on the underside in bad weather. Some people claim that the water shows the roof is leaking, but very often most of that water is condensation. Will you notice one or two extra leaks coming through the Cuben? Probably not, at least until the leaks become quite bad. For many users this might not happen for 5+ years, as they are not using said tarp every weekend of the year.
Does this mean that Cuben tarps work? Well, mostly, they do. Relax your expectations slightly, and they are fine, at least below the tree line.
PS: yeah, spelling ...