On the contrary, using a percent is more helpful
It kind of depends on the situation of how one is using it. But it is objectively true that % is simply less information than the rated miles number is, as
@GlmnAlyAirCar pointed out.
relying on number of miles (or km) is the ignorant choice, IMO,
Ah. What do you mean by "relying on"? I think I'm picking up on the idea that you think other people are looking at a number on the display like 136 rated miles and blindly thinking they will always be able to go exactly 136 real distance miles. That would be ridiculous and unwise, and I wouldn't recommend anyone do that either, if that's what you mean by "relying on".
since it cannot take into account your current and future situation.
Neither does the % display.
Today's weather, road conditions, elevation, etc is what's important, not the average conditions of the past.
Ahhhh. I see what's going on here. You think Tesla uses a Guess-O-Meter (TM), like almost all of the other car makers do for their electric cars, where that number is an adjusted averaged number based on past driving behavior and driving history. It's not. Tesla doesn't do that. That number is calculated from the car's best measurement of the actual kWh of energy in the pack and then scaled by a fixed constant related to the EPA efficiency value for that model. It's not using any "average conditions of the past".
Using the number of miles (km) is only helpful if you're driving in the same conditions as the past, like a regular commute to work
Well, no, that's not the only way it's helpful. It's helpful for intuitive human eyeballing things quickly. Humans think of how far away things are in distances, not percentages. I know my friend in Twin Falls is about 125 miles away. If I look on the display and see only 127 rated miles, I know that's not going to cut it to drive 125. But if I see over 200, then yeah, that's easy to do 125 real miles. A % number would take other calculations to try to convert it to figure out.
but if you're taking it somewhere new or on a road trip, knowing how much battery power, not how much distance, is important.
Either one can work fine. As the displays of the Trips app and the Nav screen are now, they display the arrival amount only in %, so that's fine to use it that way.
The number of miles (km) is what will get you in trouble.
You used the word "will", as if that's a given absolute fact. It's not. I've had my Model S for over 8 years now, and guess what? Back then, there was no "Trips" tab on the Energy app, and there was no % projection at arrival of your destination. We used rated miles larger than the distance miles to go to have a buffer and drove and kept an eye on it. It was fine.
I've seen plenty of reports on this forum with people getting themselves in trouble running out of energy while they were relying on % too, so it's not unique to either one.
% does have deficiencies of information. It's simply a fullness measurement. So if the battery really isn't holding as much energy as it's supposed to, it will still happily show 100% when it's holding as much as it can, even if that is a pitifully low amount. And what is 1%? 1% of a 60 kWh battery or a 100 kWh battery are very different amounts of energy. But rated miles scales the amount of energy to a reasonably consistent thing across vehicle models.
So it's fine--it's a personality preference. Some people are very stressed and bothered if they consistently see that a rated mile is not corresponding to a distance mile 1:1. And if that bothers them, then switching to % to be peaceful is a good choice.