I usually leave on a weekend so traffic to EZE isn't too bad. However, a couple of years ago, we had a huge jam on the highway because it was shutdown by soccer fans, awaiting the return of the Argentine team from the World Cup.

I tend to leave early because:

1. Traffic - I'm paranoid and don't like to stress
2. I'm usually on one of the late flights back to the US so, at some point, I'm done wandering the city and just want to get some place to sit down and relax. This means heading out around 6pm even though it's a 10pm flight
3. Duty Free Shopping - with the recent currency issues, I found that Duty Free has been a decent deal in certain cases. If you get blue rate exchange on the way in, you will get official exchange on the way out when making purchases at duty free. worked out nicely for me.
4. I find it infinitely amusing to watch people get their luggage wrapped in neon green saran wrap for 75ARS (or whatever the going rate is)

BTW - has anyone noticed that they stopped asking the infinite security questions at check-in? there's still the secondary check at US departure gates once in the terminal, but there used to be an interrogation just to get in line to check-in.