Warriors lack the potential to deal good damage with spells. Clerics are the masters of the art of healing and protection. They can heal others much quicker and better than any other profession, and they can soften opponents blows through the sanctuary and stone skin spells. They can cast spells that harm creatures and players of an opposing alignment, and they can deal out large amounts of damage to undead creatures. Clerics don't have the potential to deal as much damage with either weapons or spells as the other professions.
Thieves are the masters of the art of poisons.
They can poison their weapons, causing their opponents great pain and distress. Thieves are adept at avoiding opponent's attacks, and can cause large amounts of damage through backstabbing and circling their opponents. In addition, their ability to haggle gives them the ability to accumulate more gold than the other professions. Thieves must spend a good deal of time collecting the ingredients to create their poisons. They can't deal a large amount of damage outright in melee combat or with spells, and they must resort to good planning and good timing in order to damage their opponents.