Jürgen Hubert said:
If they are rare in-game and aren't going to have any noteworthy impact on the region in question, then why include them in the first place?
When I read a regional sourcebook, I want to know about the important stuff, not some rules about some people who nobody hardly ever meets and who are nothing special other than getting some tiny bonus in some circumstance or other.
In other words, they don't add anything of value to the book.
There are as many people who would say that the PrCs
are "the important stuff", and that the flavor text is useless fluff. Many people (myself included) pick up these books looking for useful "crunchy bits" I can adapt to my homebrew world. The flavor text is the part
I find relatively useless.
As for rarity and impact... often, the impact comes from rarity. The Knights of the Round Table, for example, could easily be an organization-based PrC. Their impact is quite disproportionate to their numbers, which were quite rare (what, 200 total out of the approximately 10 million people in England at that time period).
Even in the Realms, in cities with populations in excess of 10,000, there is a good chance that you see one or more of each PrC on the streest
every day.
Personally, I think all new classes (PrC, NPC, or Base) should have one more piece of rules information included in their descriptions: the dice roll for highest level resident in any settlement. The DMG has the numbers for the highest level resident of each class in a settlement, but it does not include PrCs because PrCs are completely optional. I know FFG has added the Thug NPC class; Green Ronin's Master Class series has added a number of Base classes; the Miniatures Handbook will add more; and countless supplements have added PrCs -- but none (that I am aware of) have addressed how many and what level live in settlements. Including that information would give a much better guideline on how "rare" they are. The formula may get a bit complex, though; perhaps requiring minimum settlement sizes before some classes appear, or having maximum sizes after which the class disappears.
In the end, those endless stat blocks subsidize the flavor text. If you separate the two, you are likely to see the end of "flavor text" supplements for a game world as they become unprofitable to produce.