the secret Morrigan/Flemeth agenda might be to "cure" the "taint", allowing GWs to live a normal lifespan and have children, maybe even with each other.
Or Avernus might come up with something. He's how old again? Granted he used blood magic, but he's rbeen estricted to more 'ethical' research.
It shows very nicely that they have a heavy price to pay.
To pay for what, though? All being a GW seems to get you is more respect so you can get things done (as far as the treaties go) and the ability to kill the archdemon. Those really aren't things most GW's would need to worry about.
Sensing darkspawn doesn't seem all that useful if you're on your guard when wandering around outsde of towns, which is a good idea in case of bandits and wild animals, too. I suppose being immune to the taint is part of it, too, but since everyone else in the game (except maybe for some people at Ostagar and mabari who actually BITE the darkspawn) can fight as many darkspawn as they want with no ill effects.