I want to look at the roles as defined by the game. First, Player roles:
Controller: Offensive characters who specialize in dealing damage to multiple foes or causing status effects.
Defender: High defense and good close up offense. They have powers that make it difficult to ignore them.
Leader: Inspire, heal, aid, and protect your allies, also has good defenses.
Striker: Deal high amounts of damage to a single target at a time.
Artillery: Do primarily ranged damage and protected well from ranged attacks.
Brute: High damage, low hit rate. High hit points, low defenses.
Controller: Manipulate enemies and the battlefield, inflict conditions on their adversaries.
Lurkers: Avoids attacks and deals high damage when they attack.
Minion: Cannon fodder.
Skirmisher: High mobility attackers, they are the baseline combat statistics for monsters.
Soldier: Low damage, high hit rate. Average hit points, high defenses. The opposite of Brutes. Have abilities that draw an enemies' fire.
They have three roles that they can have in addition to the others:
Elite: Two times as tough as a regular monster
Solo: Equal opponent of an entire party. Note that even though a Solo monster also has another role, but it must be able to function alone so the role is not as important for a Solo.
Leader: In addition to the functions of its main role, leaders grant bonuses and abilities to their allies. Unlike Elites or Solos, being a Leader does not increase the XP value of a monster.
Some of the player roles have direct analogues in the monster roles. Artillery, Lurkers, Brutes, and Skirmishers are all focused on damage dealing. This makes a lot of sense. Player roles exist as a way to facilitate teamwork. They present clear duties in combat with strengths built into the class. The DM has no need to co-operate with himself so the roles are different. Rather than a role defined as 'deal damage to one target', he should have roles defined by how he is using the monster. His Strikers need to be defined by how they deliver damage.
Defenders are almost a direct analogue to Soldiers. They have the same duties in combat, and some of the same strengths and weaknesses. Interestingly the player role of Controller is shared by Artillery and Controller monsters. The monster Controller says nothing about dealing damage to multiple foes, but instead is more focused on altering the battle in different ways.
Which brings us to Leaders. The player and monster definitions of Leaders are similar, but it is impossible to make a pure leader using the monster's roles. Your monster will be Brute (Leader), or more commonly Controller (Leader). This is unusual to me. If I were to make up an NPC Cleric focused on healing and supporting the other NPC's on his side, he would have to occupy some other role in the scheme before he became a Leader. Even more interesting, if I were to create a monster out of whole cloth that had a Cleric-like feel without using an established monster, I would need to choose one of the other main roles to base his stats on. Attack bonus, Defenses, hit points, these are all based on the six monster roles (excluding minions, of course). Adding a Leader template does not change the base stats at all.
Granted, creating a monster is less about his stats and scores, which can be tweaked to your liking anyhow, and more about the powers you pick. Still, I find it odd that there is no base monster role geared towards support. Say you want to make up a goblin shaman who heals in a different way than the cleric class. You create some cool powers appropriate for his level. He has a basic attack, and a suite of powers that heal and buff his allies. What role is he? By the book he is not a controller. By the book, he doesn't have a role. Your best bet is to call him Artillery or Soldier and ignore it from there.
My point is that the role system is great for finding and categorizing the monsters in the Monster Manual. How you use them, however, does not have to be in a rigid manner prescribed by the book. Just like player roles are flexible, monster roles do not have do define them wholly. Creating fighters who have high damage potential like strikers, or swordmages who have controller like abilities is easy and often done. Breaking the role's rigidity is par for the course, and will become easier as more books are released.
I hope you all excuse this self indulgent rant. A light bulb went off and I felt the need to put into words the mental block I had overcome. Questions or comments are welcome!