The problem is that guilds were using the apprenticeship feature to power level players. It was possible for you to have a high level drag you through very high level content and you could go from level 20 to level 60 in a matter of hours.

This was being done wholesale by some guilds to get their players up to levels where they could farm level 2 and level 3 crafting resources before other players caught up. This allowed them to get a jump on collecting resources for their guild cities (and potentially for battle keeps) before others could even get started.

Sorry, but this was a big imbalance in the game and the devs had no choice but to close it off.