Gold farmers increase the supply of money and therefore increase the price of everything. I've seen games where farmers have gone nuts and drove the price where it was impossible to earn enough gold through legitimate means to play fairly with people who have enough gold.
Worse, is that the gold farmers, especially those that use an exploit take away that area for normal players. No one can train or farm for gold legitimately, because a gold farmer has ruined the training area for everyone else*.
*An example would be a vacuum hack, which causes all items to be vacuumed into a hacker's inventory and far away from legitimate players.
Gold farmers also ruin the community, because they don't play to be part of the community.