I dunno, definitely unpopular in the sense I've never even heard or seen it before!

But I suspect it's like any unit. The one you're used to is the best. If you're familiar with joules, then it's best. Someone familiar with miliamps is going to find it best.

Seriously. Familiarity out ranks any other factor for quick understanding of a given thing being expressed in units. Doesn't matter if it's feet, meters, pounds, kilos, clicks, miles, or knots. What you know is going to be way more important than even an objectively more descriptive unit.

Now, that does mean that if everyone isn't familiar with a given unit, communication can be snafu. But when it comes to battery capacity, nobody is going to be familiar with anything until it comes up. It isn't like a mile/kilometer where having a distance be known is so day-to-day useful that even kids will pick the unit up in their heads eventually without being instructed.

For whatever reason, miliamps got picked, and that's what's on most labels. So us RC nerds and electronics geeks have become familiar with what amount of capacity is useful for a given application. Given enough time, you will too.

I don't know if joules would be better or not, since I'm not familiar enough with using them to compare, much less estimate how many joules a given battery might have. But it's no difficulty to estimate how long my e-maxx will run off of a given battery pack that's X miliamps. I used to even be able to guesstimate how ambient temperatures would shift run time per battery swap, and how it would change as the motors heated up. Couldn't now, but I can still estimate basic run time for that truck, my stampede, and a few others.

Same with power banks and a given phone.

Familiarity matters