I think when people seeing 2 year olds being ridden, they think being ridden like an adult horse. W/T/C, maybe a jump? Maybe spinning/reining strong?

I see them being ridden LIGHTLY (w/t for 15 minutes).
That's what I did with my gelding. And I don't see that as being harmful unless the rider is 200lbs+ and the horse is mentally ready.

I personally thought I'd get a bit of an advantage to the competition but soon found out/saw 2 years doing W/T/C and then as 3 year olds doing 2'-2'6 courses with flying lead changes. That, IMHO is too much regardless of a horse's mentality.
15 minutes of W/T (10 walk, 5 trot) is not going to make the horse crash and burn in 5 years.

I know a friend who has a 4 year old that is extremely green undersaddle and can't approach a jump in a straight line (atleast from the videos I have seen) yet she's showing her this sunday in the 2' class (oxers, lines, etc.). I see it as a disaster waiting to happen..

I jump my rising 4 year old occasionally but have found lately he gets too excited, rushes and then the flatwork afterwards is terrible. So that to me was a sign that mentally, he's not ready. So right now we're working on bending and using that hind end a bit more. We have a goal to show in the Novice 2' class in the summer when he's a solid 4 year old. But again, it'll depend on if he's ready. And my coach wholeheartedly agrees with that. Horses are all different, especially mentality.

What I'm tired of seeing, is people jumping down others throats for riding their 2 year old, yet they have no idea if it's just light work (like creating steering, breaks, moving off the leg, getting used to weight etc.) or if it's actual heavy work. They just jump down their throats anyways. :(