OOP: Why stop Voldemort?

[magicalme9]

No: HPFGUIDX 65646

This has been nagging at me ever since I first finished reading the 
book. Why is the Order trying so hard to stop Voldemort from getting 
the prophecy? Why guard it at the danger of ones own life? As 
Dumbledore could have told the Order, the prophecy didn't exactly 
contain a lot of valuable info for Voldemort. It wasn't a weapon for 
him after all. It just said, that either Harry or Voldie would have 
to die. Well, that didn't change much for Voldie, as one of his 
dearest ambitions is to AK Harry.
I understand that Harry had to take Occlumency, in order to stop him 
seeing what Voldie does etc., because it was clear to DD that Voldie 
might use Harry, but then, why not tell Harry so? "Harry, you have to 
take occlumency, otherwise Voldie will use you for his own purposes."
As the Order knew and the DE found out, no one except Harry and 
Voldiy could take that prophecy without getting mad, so why bother to 
stand in front of the Department of Misteries? Just to make Voldie 
think that the Prophecy was more important than it is, or to 
eventually stop Harry from getting in if Voldie would use him?
I hope this makes any sense and I'm desperately waiting for some 
answers, because I'm thouroughly confused.

magicalme (who thinks there is a severe lack of communication between 
the various wizards and if DD had told Harry why he had to take 
occlumency and what was in the prophecy a lot earlier in the book and 
Sirius had told Harry straight away what it was he gave him at 
Christmas during some quiet moment when they were alone...etc. some 
things and a certain death could have been prevented)
By the way: WHY WASN'T ANYONE ON GUARD IN FRONT OF THE DEPARTMENT OF 
MISTERIES THE NIGHT HARRY, RON, HERMIONE, NEVILLE, GINNY AND LUNA 
CAME TO "RESCUE" SIRIUS?