Gurubashi Arena in WoW, they will need to be popular and will almost definitely be driven by some form of reward system.
Arenas would have to be popular, to at least some extent. Otherwise there would be too little return on investment for Trion.
The whole idea behind adding content to an MMO is to keep people busy. They are designed to be one big time sink. Any addition to the game takes time and money to develop and the more people they can get to sink their time into that content, the better the investment for Trion.
This is why almost all new content includes some form of gear acqisition. It entices more people to spend more time playing the new content, which makes Trion's investment on that new content even more profitable.
In other words (to use a very extreme example), Trion is not going to invest a million dollars on something that will keep 5 people busy. They'll want to get as much bang for their buck as they can.