It's a play that happens way too often imo. Going at any players knees in the open field like that is very dangerous, but unavoidable often times when trying to bring down ball carriers. On blockers in the open field though, it's something I would like to see eliminated. At the end of the day, your job is to either occupy or shed that blocker. Diving at the knees is just a dirty way of occupying.
I know it's not easy for a 200 lb DB to occupy a 300 lb OT, but just because you were out schemed on the play shouldn't be an excuse to go at a players knees in the open field. I actually think the same thing on OL down field as well. They shouldn't be able to dive at a players knees on a pull just because that player has a speed advantage and they can't reach their block.