Why something must be done about pirate teams:
While initially it was only individual teams that were unable (or unmotivated) to capture a castle, there are now teams that deliberately forgo their own castles so that they can thoroughly farm out other guilds without any risk of their own. These guilds openly advertise that their members have no consequences to fear. This of course tempts them to mob players of guilds with castles that are vulnerable to attack.
These constant attacks by snipers from pirate teams leave players of affected guilds feeling frustrated and helpless. This leads to these players deciding to quit the game altogether.
Guilds without a castle only get Glory in the red zone, but not for attacks on castles. This should be motivation enough to conquer at least one castle.