Well, let's see what each system produces.

Aggro'ed mobs that will chase a player until the end of a zone allow the opportunity for:
1. Incredible and memorable escapes.
2. Acts of `valor` within a party.
3. Teamwork.
4. Chance encounters with another player that sometimes lead to lasting friendships when you save or are saved by a random stranger.
5. Sense of danger leading to greater immersion.
6. Etc, etc.

Aggro'ed mobs that decide to leave you alone shortly after:
1. Not much.
2. Boring adventures.
3. Lack of concern about surroundings.
4. Watching youtube while making periodic course corrections on auto-run.
5. Etc, etc.