"Is it the generally accepted practice to have electrically operated couplers just on locomotives and other cars that already have center rail rollers, such as a caboose?"
AFAIAC, this is the most important thing. I don't think I'd find electro couplers on everything very convenient or practical. I can see too much fiddling with the remote selecting the car and function?
I feel a scale electro coupler on my locomotives would be worth enough to make offering it worthwhile . . . meaning, I'd pay a pretty good sum for this capability.
The market was judged large enough for electro couplers to be offered in the first place! No doubt K-D asked the same "is the market there" before they ever offered O scale couplers in the first place.
People find reasons to predict failure about just about anything new! Will people pay for VL products? Do they want "scale" rolling stock from Lionel and MTH? Lots of folks will say "heck no!" They're the same folks that want to stay with the claws, post-war stuff and conventional running. Truth seems to be NOBODY knows the percentage of 3 railers who install scale couplers! Or how many would like to but just can't get up the gumption to learn how and get it done.
Of course it comes down to how much will scale electro couplers cost, how much of a job will they be to install, and how well will they work?