As long as you have CoE up on target, it will only have 0 or less fire/shadow resistance in pvp (-165).
I guess it's boring vs mage and drood that can spam decurse and you cannot longer use CoT at all but vs other classes i think it's a legit move ?
You can use those spell penetration gems/stuff to get extra sp/resilience. Am i right ?
In other words, you can use elements to replace spell pen but its not reliable.
You will always be using elements limiting your use of curse switching. Your first elements also has a high chance of resisting because you have 0 spell pen meaning you could waste time and GCDs trying to even apply elements. You now have to set up certain targets with elements before you can begin bursting, meaning your ramp up time could be increased. Curses can be dispelled which means you will be spamming elements or getting fully resisted. I can probably list 10 more reasons also.
In some situations I could see this being viable, but unless you know these situations then just gear for spell pen.