Introduction: Being able to cause massive damage is well and good, but it's a useless ability in either of the following cases: you can't hit your foe, or you've already been dropped. In the world of role playing games, these simple facts explain the existence of defenders, characters who don't rely on massive damage power to win the day, but get through their battles by outlasting their opponents. Defenders prefer to wear their foes down over time, and since they avoid most attacks against them, they are in a perfect position to do just that.
There are two primary possibilities for defenders, either or both of which might be represented in a given game's defense-oriented characters. The first involves simply avoiding attacks, causing them to fail utterly. The second involves lessening the impact of those attacks that succeed. Obviously, the best defenders are those who are able to do both.
In Quests of the Realm, defenders are represented, as one would hope, by the defend preference.
Player Tactics: The style of game can have a strong effect on the value of playing a defender. You may wish to set your sights elsewhere in some cases, because unless they have some decent secondary advantages or methods of using their abilities, defenders can be boring to play. Not only do you lack impressive offenses, but smart foes will identify defenders and avoid attacking them. Now, on one hand, this only makes you better at doing what you do best: avoiding getting hit. On the other hand, your big value to the party is to draw off enemy attacks, and if your foes don't bother attacking you, you're useless. Worse still, just because you can often avoid attacks doesn't mean you always will. If your attacks are so weak that you find it as hard to appreciably hurt your foes and they find you, you don't have any advantage at all in single combat.
In QoTR, defenders have abilities allowing them to defend their allies, thus foiling foes regardless of who they attack. QoTR defenders also have abilities that allow them to turn a foe's botched attacks to their own advantage, typically by giving them an opening to improve their next attack. If you are playing another RPG, you might want to see if you can find a character who has such abilities before you dedicate yourself to playing a defender.
On the other hand, if characters in your game tend to have multiple specialties, adding defense might be viable regardless of specialty tactics. The fact of the matter is, no matter what your other skills are, adding defense helps you stay alive, so it is always welcome. The only time you want to be careful is when considering an all-out defensive character.
On the subject of combinations, any choice can work fine, but some that stand out are attacker/defender (or blaster/defender), tanker/defender, and achiever/defender. The attacker/defender adds good damage to strong defenses, which makes for a foe both hard to kill and quite capable of killing your opponents. A tanker/defender adds phenomenal endurance to high ability to avoid hits, exponentially improving both advantages. The achiever/defender is good at avoiding attacks and good at succeeding actions (such as defenses), for a layered benefit. Such characters also have at least functional attacks to fall back on.
GM Tactics: Defenders make great boss opponents. An RPG boss, generally speaking, is outnumbered by the characters, but more individually powerful than each one. Having a defense specialization allows it to better handle the threat of multiple foes, while its naturally higher stats can still serve as offensively effective. Such an opponent can be quite stressful to players, since they get many turns but still don't seem to be causing any effect. Even if the villain isn't making much real headway itself, the sheer number of misses the characters get can make it seem much more formidable than it actually is.
Elite opponents can also benefit from a defensive specialization. A particularly effective tactic is to have a couple skilled defenders backing up more offensively-minded opponents. While the focus of these articles is always on challenging the players without putting their characters at overwhelming risk, if you are in the market for a truly dangerous battle, an attacker or blaster boss with a couple elite defender minions (provided those defenders can somehow protect the boss) makes a devastating team.
You may wish to shy away from defender fodder foes. Giving fodder opponents high defenses basically means a long, grueling, and potentially embarrassing battle. Fodder opponents are never much expected to seriously threaten the characters, but it's nice for them to have at least a degree of impact. As defenders, they just get in the way and stay there for longer.
Defenders are skilled at avoiding attacks. They have good survivability, but without other specialties, can be limited in their actual impact on combat. Consider the advantages the defenders of your game have access to. All-out defenders might have really strong survivability, but can be boring to play if all they can do is dodge really well. For players, the best defenders are those that can protect their allies. For GMs, defenders are effective ways at making a battle seem much harder than it actually is?which can be good or bad, depending on what you want the encounter to accomplish.