Gasoline is toxic but we use it every day. Chlorine and fluorine are toxic but drinking water is purposely chlorinated and fluorinated to purify and decrease tooth decay. Ammonia is toxic but is a principal component of fertilizer production. The list goes on. The point is that a substance can be toxic but safe to use, as long as the substance is handled in the right way and exposures are kept to acceptable levels.
Gasoline is kept in sealed tanks.
Chlorine and fluorine are added to water in low concentrations. Ammonia is combined with other elements to produce compounds that have completely different chemical and physical properties than ammonia. And so it is with nickel. Certain nickel-containing substances may be toxic under certain conditions but fortunately, such conditions can generally be avoided in the workplace and in the community at large. By contrast, many nickel alloys, including stainless steels, do not react with sweat and therefore do not cause nickel allergy.