On a physical level, it's actually bad for everyone. The changes in practice regarding no longer rubbing were brought in not too long ago (maybe around the last 5 years, don't quote me) but the idea had been around for a while before it was removed from guidelines. No new medical or nursing students are taught to rub, in fact they're told NOT to rub! Some doctors and nurses don't keep up with current practice though and this is why out of date information or theories remain.
It doesn't prevent scar tissue, in fact rubbing can increase scar tissue. The only way to help prevent scar tissue is alternating legs with every injection, and as for T leaking from the site, this is why you leave air at the top at the plunger and leave the needle in for 60 seconds. The last push of air after the T pushes it into the muscle away from the needle with some force and the needle staying in place lets it circulate away from the site prior to yanking it out. If you find T is leaking, then you should leave some air in the needle and leave it in longer. None should leak from the site. If it is, then you're losing a LOT of T because a lot will be lost into the subcutaneous layers and in the blood, and it's useless there. Conservation = lower dose = lower costs = win!

There really is no need to rub the area. Strictly speaking, you're more likely to lose T from the muscle if you rub. The T can end up being massaged out of the muscle into your blood, where, again, it's kinda useless. Say you have a water balloon, and you put a piece of sellotape on it and push a needle through (to create a slow leak). If you put your finger on the hole, water stops escaping or at least slows. But if you rubbed the area, even if you occasionally blocked the hole, the balloon is going to lose water at a faster rate than if you just left it alone because you're effectively pushing the water out of the hole now. Pressure on the site itself = pressure at the hole where it happens which pushes the T into and away from the site. Rubbing moves the T around inside of the muscle and towards the hole you just made, just like in that water balloon.
And also, as stated before, you've caused an injury to the skin, underlying tissues and muscle. If you did to your skin what you just did with the needle it's easier to understand how counter productive rubbing is. If we made a thin cut in our leg and started rubbing that, it's going to bleed more and potentially get deeper which leads to a bigger wound and a higher chance of scarring. The same happens to the internal structures when rubbing. It's pushing and pulling on a thin cut from the needle and damaging the underlying tissues and pulling on the hole in the muscle, possibly making it wider, and all this is why rubbing can actually lead to greater likelihood of scar tissue.
The reason people think it increases speed of absorption (not quality, merely speed) is because of the idea that rubbing increases the blood supply in the area which increases speed of absorption. And this idea is really only of any importance if you're administering pain relief. It's useless and irrelevant for T absorption because T is combined with a substance specifically designed to slow the uptake of T so that you have a more consistent T level as opposed to major peaks and troughs in levels.
If people are used to rubbing and happy with it, they're free to continue to do so. Makes no difference for me. I'm just stating the science, research and current practice guidelines that say it's more damaging than helpful and any benefits are more from a placebo effect. That way folks can make an informed decision on whether they apply pressure alone (current practice) or rub (advised against).