Adding a class level, yet no CR increase?

[Fergie]

When adding class levels to monsters, there is this text about CR:

Rulez wrote:

[Non-key] Classes increase a creature's CR by 1 for every 2 class levels added until the number of levels added are equal to (or exceed) the creature's original CR, at which point they are treated as “key” levels (adding 1 to the creature's CR for each level added).

Is it really balanced that adding one level of monk, cleric, or whatever to a monster has no effect on a monsters CR?

You get:

• +4,+4,+2,+2,-2 to abilities scores.
  
• All abilities of the class, including saves, proficiencies, bonus feats, spells, etc, etc.
  
• The benefits of increased HD, such as feats at odd levels, bonus to ability scores every 4th level, etc.
  
• Treasure equal to a heroic NPC of a level equal to the final CR.
  

And that doesn't affect the CR at all? Really?

Also, does the level added that brings the total levels equal to a monsters original CR count as 1:1 or is it the level after that? For example, if I add 3 levels of monk to a centaur, is it a CR 4 or 5?

Related rules:

Step 2: Add Class Levels

Once you have determined the creature's role, it's time to add class levels. The first step of this process is to modify the creature's ability scores. Creatures with class levels receive +4, +4, +2, +2, +0, and –2 adjustments to their ability scores, assigned in a manner that enhances their class abilities. Creatures with NPC class levels do not receive adjustments to their ability scores.

Next, add the class levels to the monster, making all of the necessary additions to its HD, hit points, BAB, CMB, CMD, feats, skills, spells, and class features. If the creature possesses class features (such as spellcasting or sneak attack) for the class that is being added, these abilities stack. This functions just like adding class levels to a character without racial Hit Dice.

A monster with class levels always possesses treasure equal to an NPC of a level equal to the monster's final CR (as calculated in Step 3, below). To determine the value of this gear, use the value listed for a heroic NPC of that level, as listed in Table: NPC Gear. Once a total GP value is determined, follow the rules for outfitting an NPC as outlined in that section. Gear should help a monster with class levels remain challenging and retain statistics close to those presented on Table 1-1: Monster Statistics by CR.
Step 3: Determine CR

Determining the final CR for a creature with class levels requires careful consideration. While adding a class level to a monster that stacks with its existing abilities and role generally adds 1 to its CR for each level taken, adding classes that do not stack is more complicated.

Table: Monsters with Class Levels gives general guidelines regarding which core classes add directly to a monster's abilities based on its role. Classes that are marked “key” generally add 1 to a creature's CR for each level added. Classes marked with a “—” increase a creature's CR by 1 for every 2 class levels added until the number of levels added are equal to (or exceed) the creature's original CR, at which point they are treated as “key” levels (adding 1 to the creature's CR for each level added). Creatures that fall into multiple roles treat a class as key if either of its roles treat the class as key. Note that levels in NPC classes are never considered key.

[HappyDaze]

The first level of a PC class grants the heroic modifers to abilitgy scores, so it's always worth a CR increase even if the class wouldn't otherwise do so.

[Frustaro]

there is some space to play with it. For example, I like to give also a warrior character level to my monsters npcs, 1hd 1 ba etc. for a +0 cr increase. I like to give my player tough fights.

[Fergie]

HappyDaze wrote:

The first level of a PC class grants the heroic modifers to abilitgy scores, so it's always worth a CR increase even if the class wouldn't otherwise do so.

Well, The Advanced Template - +4 to ALL ability scores, (and an armor bonus?) is a +1 CR increase, and these bonuses are about half that. But yeah, heroic bonuses and a HD are worth about a +1 CR alone.

[OldManAlexi]

I feel I should point out that 2 levels per 1 CR only applies when the class levels don't mesh with the creature's existing abilities. If the class levels and the creature go together, it is 1 level to 1 CR.

If a troll started taking levels in sorcerer, it would gain 1 CR per 2 levels because trolls aren't really built to cast arcane spells.

On the other hand, If a troll started taking levels in barbarian, it would gain 1 CR per level because trolls and barbarians are both meant to deal damage in close combat.

Of course, you have to be careful. You might think that because a creature did not start out with a caster level, adding one wouldn't make it that much more powerful. However, I remember hearing about how much more dangerous a group of giants is when it has access to the healing and buffs of a low level cleric. In that case, you would add 1 CR for 1 level even though the abilities don't seem to stack at first glance.