While I don't really want to speak for or against scripting versus randomly generated patterns, this whole debate is predicated on a rather interesting aspect of MMO PvE design. That is, enemies in MMOs aren't trying to win. There is an extremely basic AI algorithm and data structure called a min/max tree that could be modified to make a dynamic two agent system (enemy vs. party) that would constantly adapt to the current flow of battle and be extremely unpredictable. It's not even difficult, anyone who has taken a single class in basic artificial intelligence would be familiar with the idea. It's how your computer Chess game works.

The reason why no one ever does it is simple. Classic MMO PvE isn't about the struggle between you and the enemy. The enemy isn't trying to win. The encounter presents you with a series of challenges, and tests your ability to overcome them.

I'm genuinly curious, so I will pose an open question - and please give this a few moments thought about what it would actually entail: Would you actually like an enemy (in PvE) that predicted your future moves; calculated the optimal response and executed the most efficient way to defeat the party? An enemy that given equal level with the party, would likely win more than half the time?