Valve, at one point, used these nanites and sold them to a group of bikers. Valve appeared to be the only person whose health the nanites didn't compromise with so many pumping through he veins that he became fused to his motor bike.
The Biker gang on the other hand was being poisoned by the nanites, which could lead to the Bikers eventual deaths. Rex stopped Valve from selling them by defeating him in a race and winning his entire supply of nanites. Holiday mentioned that the nanites could be used for good in The Plant once she gets the bugs worked out of them.