Now that I think of it, if I were running D&D 3.5 back when it was in full swing, and players had monster book burned into their minds, having even a shoddy copy of a page might have spoiled some fun. But I think that time has passed.

Even if the main point of a scene is a fight with tentacles GM will reveal the true nature of later, fewer players can bring up stats from seeing which page the book is open on, you might be using some homebrew from some blog anyway, or you might even be playing a more free-form system, where even meta-knowledge of the setting does not imply what the creature will do. I think many tables back then used to be more like a boardgame, where knowing what you fight against was half the battle.

And even if some OSR fan might say now "I wish those tables were back", from what I see around, nowadays there is much more flexibility, randomness and content (i.e. monsters) is not limited to a mainline that everybody knows

So even intentional peeking at GM's notes gives much less info, and unintentional does not spoil the fun