Much like a helmet, some armor is better than no armor.
I've tried the "under armor" and found it effective, but hot. It does have the advantage of staying in place alot better than armor simply sewn into pants or jackets.
It is great to wear under leather pants than don't have armor in them. It be hard to say how effective it would have been in any specific accident but you're generally going to be better off hitting something hard wearing armor compared to not. I've seen no data suggesting it would make things worse.
When a body falls, those "pointy" parts of the body tend to suffer the brunt of an impact.....elbows, knees, shoulders, and feet. Normally feet don't have that much mass but hit a heel the wrong way and its not hard to break an ankle.
Your shoulder, and elbows can have the mass of your body right behind them, so armor will help prevent damage to them up to the point that impact pressure overwhelms any padding and things break. Having you weight over a part that's twisted the wrong way can cause bones to break with little force, so armor might not provide much assistance in preventing this.
As I've said before...You can't pick your accident. You can only try to beat the odds of damaging important parts of your body by trying to protect them the best you can....and as much as you are comfortable with.
I always wear an armored jacket and gloves. Always a full face helmet. I wear armored pants at least 80% of the time. I think if I owned some of those under armor knee pad things I'd be more likely to wear them when I just want to wear jeans....which would increase that 80% to 100.
So for that I'd say they good things to have.