Originally Posted by
Mirage
You need a completely different level design to make it possible to fight in the field. There are certain things that can't be made in levels if you want enemies to roam around. Additionally, it is much harder to program AI routines that can't be exploited by clever players utilizing the terrain to their advantage. It is much easier to keep track of this in a completely 2D grid game like secret of mana than it is in a real 3D game with 3D level design.
In short: It is extremely much more easy to program random encounters, as well as easier to design levels.