As Dumbledore said, it would be pretty hard for a mere memory to learn and plot and do new things the way Diary!Tom did. And while Tom-in-the-diary didn't know about Voldemort, the real version had given his diary-self to Lucius Malfoy to keep safe, so he obviously thought it wasn't just a worthless curiosity.

But mostly I just went on what seemed to me to be an interesting, minor tangent... which turned out to be a real -- and important! -- canon plot point. I'm feeling very pleased with myself right now. :-)