Yeah - I know that handwavium that creates biomods/devices is consumed. It wouldn't be grey goo, anyway. The really scary thing about gray goo is that it consumes the stuff you care about. You know - your house, your car, your pets, your family, your body below the waist, and so forth. Even a completely runaway version of handwavium wouldn't do more to the stuff we care about (people, cities, ecosystems) than alter things in bizarre (but largely nuetral-to-moderately-beneficial) ways. I'm not even saying that it should or would necessarily be happening at any given point in the plot - just that there are people who might worry about it happening, and that those worries aren't entirely unreasonable.
In terms of intent, I always figured the handwavium was generally guided by intent to a degree, but not always. Sure, maybe 99 out of 100 times that handwavium gets thrown away, it goes mostly inert. It's still not necessarily a good idea to hand out 1000 Happy Fun Balls to the children of NYC and wait for them to get bored. It's also a Really Good Reason for some people to worry about handwavium contamination (even if it *usually* doesn't go much of anywhere) and lets some of the handwavium-unfriendly governments of the world be a bit more on the Decent Human Beings Who Disagree side of things, rather than just being a bunch of power-blinded fearmongers or whatever.
In terms of intent, I always figured the handwavium was generally guided by intent to a degree, but not always. Sure, maybe 99 out of 100 times that handwavium gets thrown away, it goes mostly inert. It's still not necessarily a good idea to hand out 1000 Happy Fun Balls to the children of NYC and wait for them to get bored. It's also a Really Good Reason for some people to worry about handwavium contamination (even if it *usually* doesn't go much of anywhere) and lets some of the handwavium-unfriendly governments of the world be a bit more on the Decent Human Beings Who Disagree side of things, rather than just being a bunch of power-blinded fearmongers or whatever.