Custom Arenas are for practicing in a controlled environment. Which you cant currently do.
Another thing, Blizzes system for arenas does not work. If it did you would not get players constantly creating new arena teams each week when their rank gets to high. If they had a good system even if you left a team you would still get matched against people your level. Anets system does this. Also Arena combat isnt the same as GW2 spvp. Rated RBG's are more like it. And blizz doesnt have Random RBG's so cant really compare the two. Blizzards game is no where near balanced, tons of specs are not viable for pvp.
The charge for these rooms does not matter. Gold to gem exchange means you dont have to spend irl money. Also guilds can pool money to make their own to practice in. The life can be extended by anyone with access to the room
Also think about it from Anets perspective. Spvp only players prolly arnt spending that much money on gems. This could be a good way to tap into spvp players pockets for something specifically related to Spvp