The major benefit of the event is a) knowing exactly what item you will be getting, and b) being able to eliminate some of the RNG from the rolls while farming. So, you want an Austringer, and you're looking for a max range roll? So you drop a Rune of Desire in slot 1, and a red rune in slot 2. Add a green rune in slot 3, and every time you open the chest, you'll get an Austringer with a range masterwork, 100% of the time. Now you are just chasing Rangefinder, Accurized Rounds, and Outlaw / EotS. Normally, if you run an activity, you have a 1 in (however big the loot pool is, plus rarity modifiers) chance of even getting the item you want. Now, with chest farming, you can guarantee yourself 4-6 rolls of the item you want, and additionally know that one of the perks is exactly what you want. Speeds up gear farming emmensely.

One of the perks refunds a rune every time so you can grind a weapon type at no cost. Got a couple of Erentils and the new fusion doing it.

Good point. I would say it's easily the most fun co-op in D2 :)