The real key to understanding how the MM3 monster entries were written lies in
this DDI article, for those with access. Regarding the In Combat section, this is the essence of it:
Mike Mearls: The tactics section has transformed into the “… in Combat” section to give writers more flexibility. It can talk about specific tactics, illustrate roleplay hooks, present storytelling methods, and offer other interesting ideas for bringing a monster to life in the campaign. Think of this paragraph as advice on the best way to make the creature an interesting foe. That can be uses of a power combined with specific terrain or tactics, general guidelines on how these guys prefer to fight, or roleplaying advice on battle cries or how to otherwise depict the creature. It should reveal something about the creature’s nature and how that nature is reflected in combat. As an example, the skulk entry highlights how these creature go after downed foes to satisfy their bloodlust and ties that into their hatred for civilization and cities. That’s a unique tactic which grows out of the skulk’s backstory and makes them unique foes.
So, when looking over the In Combat section think of it in those (flexible) terms. In the banderhobb, nymph, and yeti entries, that section illustrates the mood, feel, or tone of one or more of the creature's powers. For those particular creatures it's a narrative aid to key the DM in to how this creature feels or behaves in combat or how one or more of its powers work story-wise. It's really just a quick illustration of a power or two, to communicate its mood--
not an arbitrary piece of short fiction.
In my experience, a monster's tactics are greatly influenced by the encounter's terrain, its allies in the encounter, and whatever is happening in the story. Since the new stat blocks give you a pretty solid idea of what you generally want to do with the creatures, I chose to go the illustrative route to show how the monster and/or its powers behave rather than write a script for what attacks it uses and when, since in games I've run those tactics have often been rather circumstantial. I understand that point of view won't make
everyone happy, but I'm not sure there's a right choice that would (heh, though not for lack of trying, believe me). Since the Compendium and Monster Builder are my primary resources when putting an adventure together, I typically consult the Monster Manual for a creature's story and flavor--and that's why I tended to lend more focus to those mood, flavor, and story aspects.
@ Scribble: the dark master and history of the banderhobb is for you to determine. For me, the ambiguity made them creepier. Perhaps someone will define those things in a product someday, but I don't know that anything can ever be as cool as the way a DM interprets them and sets them in his or her own campaign. By way of example, I think I was happier with LOST before the show started answering questions, especially if I found out I didn't like the answers.
That's my three and a half cents on the why's and the wherefore's of MM3 "fluff," at any rate.