It's not necessarily an apprenticeship/trainee type thing. Every team has a different balance to every race. If we are taking the TdF as an example - teams tend to have climbers, sprinter and GC contenders. The term domestique literally means servant, helper. In the case of USPS you essentially have 8 helpers for one GC rider. Many of them have been specifically employed to help Armstrong win the TdF. Heras though may again try and win the Vuelta this year (after a runners-up last year).
Tyler Hamilton moved on from being a helper to Lance to a genuine GC contender at CSC.