Her blood is the same as Dawns...but Buffy is not the key. Her body does not contain the mystical energy that Dawns does. The rule about closing the portal was, once the blood flows no more (the body giving the blood is dead), the portal will close.
Why didnt the monks destroy the key Buffy?
The monks didnt destroy the key because they wanted to use it, perhaps to bring enlightened beings from better realms or to travel to better places. The key is otherwise described as being indiscriminate, merging all dimensions, so maybe they would want to do some further refining of their spells.
What is the key for in Buffy?
The Key was an ancient mystical energy with the power to unlock the gates between all existing dimensions.
What is the deal with Ben and Glory?
Glory was too powerful to be completely contained within the human vessel. When Ben reached his 20s, Glory began to gain control over Ben for short periods of time, taking the form of a vain, self-centered human female possessing superhuman strength, speed, and invulnerability.