There is currently NO REASON to use OPERATORS outside of plains and kuva.

[Ardhanarishvara]

People keep missing the point of Operators. There's a few in this thread that get it, and a few that don't. Allow me to spell it out for you.

Operators don't exist to be used instead of Warframes, They exist to be used in tandem with Warframes.

Yes, they have okayish weapons now. Yes, there's a few nodes that make it possible to use them in lower level content without issue.
No, they do not exist to replace your Warframe.

The Eidolon fights are a prime example of this. If Operators were intended to replace your warframe, you should be able to kill the Eidolon with only their Amps and abilities. You can't. At least, not nearly as easily as you can with typical weapons and abilities. You'd be able to use the Railgun Amp to shoot off kneecaps and biceps but you can't. Not without aid from other weapons or warframe abilities. That should make it clear what the Operator's role in combat is. It's to create openings. It's to cover gaps in your playstyle or facilitate further advancements. The two most obvious being to battle energy consumption for casters and, for me at least, melee combo counter drop off for brawlers.

Some people seem to have this... odd sentiment like, why would you use Void Radiance to blind enemies when you could just bring Excalibur? Maybe you don't want to bring Excalibur. Maybe you feel like using a different 'frame today. It needs a little more setup, yeah, and it's a shorter radius, but you also happen to be invincible while setting up, so why not. Especially if you're already using Naramon for its ludicrously amazing passives, or for finishers from Executing Dash.

[andwar]

I've mainly been using the operators as a pseudo teleport. Since DE made it possible to seamlessly switch between operator and warframe, it can be used to extend your jumps, as a gap-closer, a getaway tool or quickly changing directions in midair. I understand if having this added option isn't very useful if it doesnt suit your playstyle, but I think its a nice addition to have nonetheless: useful but not mandatory.