In BD the enemies in the battlefields did not respawn (or in the main game), but for the after-game battlefields the fields themselves 'respawn' (from the temple you could enter rifts to randomly generate new battlefields).
The game is presumably balanced taken account of mindreading, so the benefit of what you can learn makes up for the slower leveling. As you say, we don't know if it is intended to be used judiciously (mostly on NPCs involved in quests) or fairly generally (like farmers in DD occasionally having high level items for trade, minor characters in D2ED could know of treasure they are too weak to retrieve, or servants could have overheard their employer's conversations, etc).
Respawning opponents would be nice to replace any experience 'wasted' on useless mindreading attempts, even if that isn't really necessary (in BD I skipped the opponents and quests in the BF, in part because shortly after the game was released some people complained that you had to level up in the BF to be able to survive in the main game).
