Sure we can argue that. If we're under the cloak of Fair Use, well, Fair Use doesn't care whether somebody _else_ is using another portion of the book, or a million somebody elses are using a million separate portions that might together constitute the whole, so long as each individual use is Fair Use, then we're good. And if we're not under Fair Use at all, well, then we're screwed no matter what we do, so unless we want to pack up and quit entirely, we might as well have fun doing it our way.
As I recall, the current rule is 1/3, or 5 pages for new issues. I already think that's too little, but most previews are at least 5 pages, and many are 7, which means under your theory nothing but previews can ever be posted of a new book (if it has previews, but it's been my experience that most of the books of the Big 2 get their usually 7-page previews). And that strikes me as way more unweildy than a 'no previews' rule. Posting to scans_daily is supposed to be about glorying in the awesome (or the reverse), and companies rarely put the awesomest bits in the first few pages.
Frankly, if preview pages count even if unposted, then I've got no reason to come here. I can find previews easy enough for myself, and most comic sites that have them have forums that you can discuss the previews if I were so inclined. It's just not worth following the com just for old stuff that may or may not appeal to me at all. As it is I think the reduced page count rules have been a severe detriment the community.