...and finally you would need to clean your case out often as you pretty much created a vacuum cleaner.
This isn't really the case. I have a completely open bottom intake, no filter at all, and after not touching my case for almost a year I had some dust inside, but not enough to be causing any problems. And I'm a horrible housekeeper, so it's not like the thing is in a dust-free environment. I daresay any case regardless of intake position or filters would have some dust in it after a year. A much bigger issue imo is that in a cluttered environment things could get piled around the base of the computer, starving it for fresh air. In a one-off case like mine, that's not a big deal, because I can just be smart enough not to clutter there, but a manufacturer has to think about things like that.