In a sourcebook such as Waterdeep, the whole point is to give as detailed information on the people, places, and history of the subject as possible in the space allotted. I am baffled by the attitude that more variety and creativity is a bad thing when writing source material. The purpose is to flesh out the location sufficiently for DMs who do not care to spend the time necessary to do so themselves. I feel that NPCs are more unique and interesting with the wide variety of options available to the writers. Here's a few examples:
Muiral the Missshapen: tauric human/large monstrous scorpion barbarian 3/wizard 14/archmage 2
Trobriand the Metal Mage: wizard 12/techsmith 5/archmage 1
Mhair Szeltune: wizard 5/guild wizard of Waterdeep 10/archmage 4
Kappiyan Flurmastyr: wizard 7/master alchemist 10/loremaster 3
The vast majority of the NPCs in this particular book are only listed with their name, race, alignment, and class, so if I don't have Magic of Faerun and I want to use Kappiyan Flurmastyr in a game, I can easily make him a wizard 10/loremaster 10. If I have a PC who wants to join Lathander's Order of the Aster and I don't have Complete Divine, I'll tell the player to take Improved Turning instead of Disciple of the Sun. Please don't try to take away the writer's ability to convey more info about a NPC or group through their choice of classes and feats.
Now, if we're talking about an adventure module, I completely agree that references to outside sources should be kept to a bare minimum. Where at all possible, the authors should include as much relevant information as necessary to do so. In a sourcebook, I just don't see the problem.