Exactly. Why photograph it and then release that photo to the public? If it's determined that it's of no use, then why release it to the defense team, and, consequently, the public?
If they thought enough of it to photograph it, then I think that they would've thought enough of it to test it. By releasing that photo yesterday, a large part of the people following this case went nuts. If in fact the age of that entry can be proven...wow. She's freaking toast. Premeditation, lack or remorse, sheer joy over killing and disposing of her daughter, all in one nice, neat, one page little package. Why would the SA risk messing up like that if the journal page meant nothing? Like other posters have said, it could affect the outcome of the trial.
I trust that LE has their reasons for this, and I think that there was testing done. We may not see those results until trial, though. Fine with me. I can wait for something that good.