The promotional Diablo event made one thing fairly clear: people will inevitably find the pattern to these events and swarm them constantly. Maybe slightly less so over time, depending on the loot, but it still makes for a terrible experience for the capital cities, where events such as the Diablo and Hearthstone loot piñatas went.
This is especially a problem in RP servers, where the cities are intentionally left unsharded (which is good!) only to be forced into shards because they become impossible to be otherwise (which is bad!).
The Hearthstone event was placed in Durotar and Valdrakken, which is an improvement over the Diablo one – so why on earth did the Alliance version still go to Stormwind???
Even with things like the Rumble promotion, all this does is further crowd the capitals with nonsense, especially in the case of Orgrimmar, where everything happens in the Valley of Strength.