Copper is used to avoid [lithium-halogen exchange][1]. Organolithiums will often, instead of acting as a nucleophile, form a new organolithium: $$\ce{RLi + R'X -> RX + R'Li}$$ Using $\ce{Cu(I)}$ allows the formation of an organocuprate, which acts as a nucleophile, allowing alkylation rather than metallation: $$\ce{2RLi + CuX -> LiR2Cu + LiX}$$ $$\ce{LiR2Cu + R'X -> R'R + RCu + LiX}$$ I'm not sure if silver could be used instead; I'm not seeing any refrence for it, but I may just not be searching the right terms. The [Wikipedia][1] page on organosilver chemistry isn't terribly extensive, but I'm guessing stoichiometric use of silver, while not terribly expensive as transition metals go, would certainly be more so than analogous use of copper. 


  [1]: https://en.wikipedia.org/wiki/Organolithium_reagent#Lithium-Halogen_exchange
  [2]: https://en.m.wikipedia.org/wiki/Organosilver_chemistry