I'm wondering now how much cc the pet is susceptible to. I would think stuns, knockbacks, fear, cyclone, death grip but I don't know about a lot of other stuff (not saying it isn't susceptible to that other stuff I just don't know). And some of that cc are cooldowns. I can't imagine that a pally for instance would decide to HoJ the pet instead of me just to interrupt the channel but hey, if they do that's fine with me. The other thing is the range on the totem is something like 60 yards (i'm estimating on the shorter side) so you can move the pet (and the totem if you have projection) out of LoS and have it passively stay there while it channels (the channel has a 40 yard range). If you outrange the channel on your own or because you were cc'd out of range, or if you're in range but cycloned, for example, the channel will stop. So I don't know how easy it would actually be to interrupt the channel or force the pet to move, some classes would be better doing this than others.
Some other downsides to using Primal EET is that you wouldn't be able to use earthbind or tremor totem. I'm not mentioning SBT or earth grab since I think if you choose to use Primal Elementalist you're better off picking one of the other talents in those trees so you that will lessen the chance of having to use another earth totem and override your elemental. The upside in using this in arena is you'll know who you are facing before the match starts, so that if the enemy team doesn't have a fear then you can feel relatively comfortable using this. You can even be relatively comfortable playing without earthbind since the pet has a 4 second stun that you can use twice in one minute(stun has a 40 second cooldown).
There are probably any number of other downsides but I think the Primal talent could be very nice depending on what arena comp you're facing.
These words in my mouth... where did they come from? I don't think I'm the one that put them there...