The point is that WHILE it is secret, it's fun to endeavor to uncover it and understand it. Just because someone knows it's their birthday doesn't mean you DON'T shut out the lights and hide before they walk into the surprise party.
Well, I think the real problem is that for many GMs, the magic items in the DMG and the monster stats in the MM transform from meta data to instance data the moment they get put in the adventure. Kept the exact same, if the players read the meta data, they know more instance data.
The solution is obvious, GMs need to make up new monsters and items from scratch. But that's more work, and defeats the purpose of buying books filled with monster stats and magic items.
Hence the trade-off and risk.
Ignoring the issue of players knowing monster stats and magic item descriptions, is there any other stuff in the DMG and MM that hurts the game if the players know?
If the players know the rules on building NPCs and Monsters, they can try to nit-pick on whether the custom NPC or monster is built to spec, but technically, they should never see the stats to prove anything, and thus have no case.
The biggest problem I see is rules lawyering, particularly in questioning everything the GM builds or rules on, just because from what the player sees, it looks like it breaks a rule or guideline.
I would argue, that this is solveable with a rule that says "the rules are tools for the GM to build an adventure that is fit for the PCs, and is not subject to scrutiny by the players". Until there's a point buying system for the GM in building the world, adventures and encounters, that has to add up and can be audited by somebody, the GM is effectively making it up as he goes along, and the rules are simply tools he uses to add a layer of authority.