I do agree that 0.74oz cuben can be successfully used as a floor when taken very well care of. I do think it needs very good care though, and not just the good care that more UL'ers try to practice.
When I say I'd prefer a 1.2oz cuben floor, I'm mostly speaking from my experience with 0.74oz cuben stuff sacks. Absolutely the stuff is entirely waterproof when it's undamaged and can handle 31 days of rain. My concerns lie with the abrasion and poking damage that might be incurred when used over the long term as a tent floor.
When I choose a tent site, I spend 1-2 minutes picking it free of obvious pinecones, sticks and rocks, but I don't get every little bit off so there's always a chance of a small sharp rock or a pokey stick. Based on my experience with using 0.74oz cuben over the long term in a stuff sack application, I think opting for 1.2oz cuben for the floor would be weight well spent.
I never thought my 0.74oz cuben stuff sacks would see too much wear from just setting them around camp etc, but after 1 year of regular use they had numerous pinholes so weren't nearly waterproof anymore, and after 2 years of use there were real holes developing to the point that I began to be concerned I'd lose small items out of the sack. Last night I replaced my main ditties sack with a new one made of 1.2oz cuben. The former sack was 0.74oz cuben and bonded using 3M tape. Despite using tougher cuben, the new sack weighs less (5.0g vs. 6.7g) because I sewed it instead of using adhesive tape.
Anyways, I think my stuff sacks have seen a lot of high wear since I've never really been too concerned with taking care of them like I do some of my more valuable gear. I know this is the SUL forum and we're focused on SUL shelters, but I think an extra 1oz for a tough floor is weight pretty well spent. A typical solo shelter might have a floor area around 2 square yards (ie. 30" x 84"), so the extra weight of using 1.2oz cuben would be 2 x (1.2-0.74), which is 0.92oz. You could easily save that back using 0.51oz cuben instead of 0.74oz cuben for the canopy. I'd much prefer a 0.51oz canopy & 1.2oz floor, over an all 0.74oz shelter.
Photo of my 0.74oz stuff sack bottom after 2 years of use. Duct tape is covering the largest hole: