Considering the items we call Exotics offer great power at the toll of suppressing your light it could very well be an oppression method. That's just my take. Look at it like this, they are not our immediate enemy- however, being that guardians have stopped 'gods', if they were to oppose the nine they could very well throw some kinks into the nines secretive agenda. So, if they were to offer us these abilities at the cost of our own light- would it not make it easier to destroy us in the long run if need be? As for the Strange Coins, I believe they are a key of sorts. They emit a strange humming. Everything in our universe has a specific resonance pitch, just as, if you traveled to alternate universe it would have a specific pitch. I think its plausible the Strange Coins are a key to a gateway of sorts and we are unknowingly giving the Nine the keys to the Lamborghini.