"Some" is what, a few? Less than "many", certainly not "most".
I'd say keep it legal.
But we usually have either most-legal, some- illegal (PVC pipe than can be used to make bongs) or some-legal, most-illegal (Tommy Chong's bongs).
Can you give me an example of a dual-use product that is some-legal, some-illegal?
It's a good question -- quantifying this is exactly the problem. The media hypes something, and all of a sudden the banning hysteria kicks in, with no rational investigation about whether the product is being used mostly for legal purposes. To come full-circle, hemp was mostly legit use before it was made illegal. Even though one can apparently extract hash oil from it, it was much more widely used in textiles. So by your criteria (mostly used for legitimate purposes), it shouldn't have been banned, correct?