Double floors in my experience trap condensation between layers and also can be a pain if the two get out of alignment or the polycryo gets bunched up underneath. It hasn't worked for me at least.
I've used a 30D PU coated nylon floor for the past 10 years at least. Other then the last few years the floor has been extremely bomber (recently the PU coating has started to disintegrate and peel off, but we're talking tiny pin hole areas).
I've yet to read an account of a silnylon floor failing in terms of waterproofability (assuming reasonable abuse). I've heard of misting from above only on certain tent/tarp canopies made of lesser quality (apparently US made) silnylons, the higher quality ones (ie. thru-hiker Shield, which I think MLD and TT use now if I'm not mistaken) I haven't heard of misting except in some of the most extreme conditions. Cuben on the other hand, I have read accounts of failure in stuff sacks and floors. I've gleaned this from this site through significant lurking, but sure, more accounts of cuben floor users would help.
Other than 1.5 ounces, less slipperiness, the hydrophobic nature of cuben (really how wet is your floor getting even if its raining if you're under shelter?), I don't see added benefit for going with a cuben floor. Even though you can tape up cuben in the field fairly easily, I wouldn't really want to do a repair in the field without alcohol to get the surfaces clean and to do it right. In the field, I much prefer the piece of mind of just not worrying about the silnylon floor as much because it is that much more durable and relatively cheap to replace/repair if something drastic does happen.
To those who have voted for the cuben floor, what added benefits does the cuben floor give you other than the three and a half I just mentioned? Are those added benefits significant to you? Can you give some examples of specific conditions in which those added benefits made an experience noticeably more pleasant?