"A 10,000mm rating on the floor is completely, utterly unnecessary unless you decide to set up your tent in a puddle. It is a marketing tool by MSR to make sure that potential consumers are not turned off by the thinner material now used on the floor of the Hubba models."
1000mm is the bare minimium something can be rated at and still considered waterproof. If it's under 1000mm, it generally can't stand up to a couple hours of light rain. 1200mm fabric is cutting it awful tight to this IMO and is on the edge of what can stand up to continued heavy rain. I haven't done any scientific tests, but I bet you are close to exceeding 1200mm of pressure when you walk on the floor of your tent or press the tent floor between your knees and rock as you crawl around.
Making things tougher is that the waterproofness of fabrics degrades over time. The silicone on the nylon wears just by using the tent....not to mention the considerable wear that soaps have on the silnylon if you ever need to wash it.
I'm not trashing silnylon here or saying it won't keep you dry, but I do think people need to realize that the waterproofing on silnylon can fail. It's hardly 'completely, utterly unnecessary' to have something better. 5000mm is pretty much the lowest rating you can buy a waterproof jacket at. I think we've all had 'waterproof/breathable' jackets that have become not waterproof after a few years of use, and these jackets all started off way more waterproof than Silnylon.
"[A 10,000mm rating] is a marketing tool by MSR to make sure that potential consumers are not turned off by the thinner material now used on the floor of the Hubba models."
10,000mm is pretty much the standard floor waterproofing from all major manufacturers. It has nothing to do with MSR trying to fool their Hubba customers. You can find the occasional tent from major manufacturers with a 5000mm floors (generally only ones aimed at summer use) and once in a while a 2000mm floor but 10,000mm is by far the most common.
I'm not arguing that 10,000mm is necessary, but I do feel that 1200mm is on the borderline of what you can get away with and there isn't much margin for wear over time or if you get stuck camping in muddy conditions. I would be fine with a 5000mm floor and I might gingerly use a 2000mm floor but 1200mm makes me pretty nervous. I work for Helly Hansen in the warranty department and I see our old 2000mm waterproof jackets (aka HellyTech 2) coming back all the time now that they are 4-5 years old.
For examples of 1200mm tents failing, check out this link which contains reviews of the Big Angles SL2. Here's a few tasty excerpts:
"During 24 days of hiking, I slept through about 12 nights of rain. If you're part Springer Spaniel, you'll love this tent (read: WET)."
"Anytime you touch the walls or ceiling while the fly is wet, water leaks right through, and if you have 2 adults in this tent, your sleeping bags will touch the walls. Our sleeping bags got soaked in this tent. Water also pooled on the floor"
Okay so this is a darn long post. The bottom line is that 10,000mm may not be necessary but it's hardly a conspiracy by MSR. 10,000mm floors have a much larger margin of safety than 1200mm floors, which can fail as they age and/or in really wet conditions. It seems like a lot of people on this site are biased against major manufacturers...10,000mm is clearly safer than 1200mm. Can't we just acknowledge that this is one advantage with the MSR and move on to other topics? Or if you really want to discuss it, put some facts behind what you're saying instead of just dismissing tents higher than 1200mm as ridiculous. The reason I'm responding to this isn't because I want to argue with you, but rather because I'd feel bad for the layman who is fooled into thinking anything over 1200mm is unnecessary.
Closing quote from HammockCamping.com:
"However, the silicone is slightly water-soluble and repeated long exposure to rain, or repeated washings, can render the fabric less than completely waterproof."