- More resource generation.
- More lifestealing.
- Enemies just get in the way of each others. It is quite amusing how a few highly mobile mobs in act 2 can get you in trouble, but the hordes appearing in act 3 simply don't have enough space to move and simply get slaughtered.
- In the case of wizard at least, shorter CD (Crit Mass), and more Archon uptime.
On top of that, of course, more monster mean more gold, experience, gems, and a better chance to get a legendary (though it will suck anyway...). So, tell me: What mob density would you choose? If you tell me anything that is not "as much as my PC can handle!", you are a bad player, a terrible layer, or both.