Constant instance variables (or 'instance constants'? :p) would be terrific in their own right. Even so, I think traditional 'Enums', for me, excel at 2 important things that make them especially worthwhile compared to other approaches:
1. Forcing a single choice. An enemy can't hide and attack at the same time. In code, instead of having to specify a priority order, one could simply select 'State.HIDE' from the list and call it good - super clear and less prone to buggy logic - like using a Boolean instead of isTrue and isFalse vars (hopefully no one has ever done that). Plus, the idea of a neat little drop-down in the C2 user interface when testing against an Enum really excites me.
2. Less prone to typos. True that Javascript enums are just strings, but I guess there's also the possibility of using the Object.freeze method to seal a property into an effective Enum, eliminating the typo issue. I feel like in many cases in JS games, this would be more desirable than using dynamic JS enums. Likewise, in C2 having an Enum in many cases would be preferred to using strings or constants.
All in all, I think Enums would be an awesome addition to have down the line, but I see why this wouldn't be an immediate priority because there are a handful of alternate approaches.