How can organocuprates generated with catalytic Cu(I) be competitive in 1,4-addition to an enone?

Organocopper compounds (e.g. $\ce{LiCuR\_2}$) are often prepared from an organolithium or Grignard reagent when a soften nucleophile is needed, such as for a conjugate addition to an $\alpha,\beta$-unsaturated carbonyl. Clayden et al. (p.499) recommend generating it *in situ* catalytically via transmetallation with 1% $\ce{CuI}$. 


My question: if at the start, there is 1% of organocopper reagent and 99% Grignard reagent, why don't those 99% do direct addition? Why is the organocopper reagent so much more reactive that conjugate addition is the main outcome of the reaction? 


