Ron emails us a very simple suggestion as to why the Forerunners decided to examine - and not wipe out - the Flood.
Ron Kobish (firstname.lastname@example.org) writes:
I Think I know the reason that the forerunners kept the flood for a simple reason.
Not as a cold war weapon or to keep out the poor people in some kind of fortress world.
They could do either easily with their knowledge of engineering, physics, chemistry, etc.
But They kept the flood because it was the one thing they knew nothing about and as a highly advanced race, they could not have that.
The Reason they kept the flood is because of a thirst of knowledge and simple curiosity.
Sounds logical to me.