It isn't incriminating in the scenario that all was rosy and grand and there was no fight and no strife in the family at all and that Casey was getting a grip on her life. It is evidence that supports roses and sunshine. So why stop at that page? Why not destroy the entire existence of the diary if there was anything in there to destroy in the first place?
This has puzzled me for a long time, and I can only come up with one reason....IF Cindy is the one who tore out the pages (and there were pages torn out to be sure), what was on those pages were about her, and no doubt very unflattering. I imagine it a lot like feeling as though she'll show Casey that she's seen what she wrote about her, just so Casey knows she has no secrets. She'll probably never say a word about it, leaving it to Casey to wonder when the other shoe will ever hit.
However
IF it was Casey who tore out the pages, then I imagine it was done so because she erased a part of her life. Pages that talked about her time with RM, mentioned other guys, discussed her her her. And no doubt they were torn out because she had been slowly moving herself into Tony's apartment and what if he found it and saw what she wrote???
The only struggle I have with this diary at this moment is Cindy's chops-licking look at BC last night on LKL when the discussion of the diary was brought up. Maybe she has read "blogs" about the discovery items, this diary in particular. Maybe she does feel like she's pulled one over on everyone since there's nothing there and nothing that could possibly be discovered, or so she thinks. That she feels sooo smug that BC has that diary tucked away for safe keeping.....well, maybe she's smug about nothing, but she thinks it's something and in true form, it would be equally enjoyable for her to make it seem like it is, when it isn't. That's when my head explodes and I have to spend 40 minutes or so cleaning walls.