My guess, is that it makes you become a symbiote. An unused tadpole would be closer to a parasite.
The best spies are both perfectly human and perfectly in tune with the Absolute. If you don't merge, it can only ever attack your mind. It's much more insidious when you've become one entity. Perhaps, you usurp the Absolute in that route instead of saving the world from it.
It might be interesting to see how different origin characters handle that role. It's also technically a special ending for each character and explains their playability a bit
