Ye because RBGs havent been plagued with bots due to them being able to auto target the same person faster than an actual player could ever do or insane class stacking mmm I am sure thats why Blizzard gave up on large scale pvp a long time ago if not where is the RBG tournaments at Blizzcon or at all
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As for large scale PvP plenty of other MMOs have tried and all have pretty much failed on this aspect it becomes a shit show of even less balance that what current wow has with ofc the same issues RBG has with extreme class stacking and tactics being boiled down to your guild leader telling you to attack from the south and making sure to kill everyone from x class to we need to move as a unit. This I say as someone who has been part of PvP fights in other MMOs where the have been almost 100 people on each side of the battlefield hell sometimes there being 5-7 guilds there with that many people in each guild on that note have fun with your slideshow game.
So thats why they REMOVED 5v5 all together from arena due to it being the most balanced format and that everyone played and loved it
on that note is that also a majority of pvpers do RBGs by far it being the most popular form of PvP ever crated and not the for mentioned disaster