I think there is more that can be said. May I pick up from here^ ok? ... Thanx!

In direct response to the question you first posted Aaron, The real AI behind the enemies, isn't the awesome programming for the monsters, it's the intelligent and creative design that went into the level creation to take advantage of those monsters' abilities. Today's monsters don't play on an equal footing as the player, meaning, they have different weapons and abilities that the player doesn't have. Ex. Strider vs. Gordon Freeman, not a fair fight, but it can be made fair by building a balanced level. (and positioning appropriate weapons in working spaces).

Of course, your question applies to the theoretical idea of having a blank room with vary difficulties of enemies and asking which would you rather have: more less harder ones, or fewer harder ones? But truth is, it's pointless to think like that, because you're negating the entire level experience. One could be more fun than the other if given the chance to bloom in an awesome space created by a level designer.

Level design is the real game AI, as it sets the stage for an encounter with any kind of enemy. It's not the difficulty of the monster, it's the difficulty of the level. And guess what! No programming required! You can make a monster WAY more difficult in fewer numbers if given the opportunity to show his stuff in a level. Or you can make many monsters that are equally difficult, by giving them multiple stage entrances to perform from.

I would say that Portal displayed AMAZING difficult game AI without using any 'monsters, ' just simple game mechanics and level design. (of course it's a puzzle-type game, so this is to be expected).

Throwing in monsters won't make a level any more 'fun' by guarantee. It's how you use them in your levels, if at all. So the question whether there should be more less-difficult ones, or less more-difficult ones, is pointless, because it changes from level to level.

I personally don't like using many AI enemies at all if I can help it. I use them as story-telling devices, and ways to help increase action. Although I guess it can be argued that most any action scene in a current game today will have NPC enemies at their heart. I think of them as a cheap way to insert higher amounts of action into a level. IMO, they're icing on the cake; in most other opinions, they're key to a game's development and story.

Use them how you'd like!