For the upteenth time, if you read the posts they are not adding Spectator Mode to pet battles like you think. Instead of seeing the person just standing there now, you can see the pet they are battling and what they have out.
Spectator mode when mean you see the action as it happens, but you don't. You just merely see two things going at it and it's always been like that just that it was "phased".
Stop using the pet battle spectator as a reason why it should be added to Arena and stop using it as an excuse of why it hasn't been added.
They said spectator mode for arena would require a lot of work. It would require a lot of code, figuring out how to do it, etc. Also, exactly what is the point of spectator mode when you can just watch them stream?
Also, while you may want it, the percentage of the player base that would actually use this time consuming project would be very minimal and resources are better used elsewhere.