Quite frankly, random encounters and respawning monsters never really work in a single player or story driven RPG. (the only game that managed to pull this off THAT I PLAYED is Diablo).

The thing is, each battle is designed and scripted individually to be a unique challenge. The amount of monsters, and with that the amount exp there is to obtain, is highly tested and tweaked up until this point.

For instance: players shouldn't be lvl 10 by the time they try the lighthouse battle or it would just be too easy and thus not fun or challenging at all. Respawning monsters could lead to grinding, players being OP at bosses and by doing so taking away most of the challenges that are within the combats.