The link is dead now, but this is the published minutes from an ABC rules committee when they were discussing linear strikes to the knee:
Two doctors, one an orthopedist, said it wasn't especially dangerous.
It also doesn't seem to have a history of insane knee destruction in striking arts where it's legal.
Thanks for posting that, Smacky.
Who cares how dangerous it looks
; the EVIDENCE so far indicates that it simply isn't.
Let's all help put the combat back into our sport by ending all the hand-wringing over potential injuries to fighters.
These are grown-ups who signed on, fully aware of the rules and the risks.
In addition, you can't have an omelette without breaking a few eggs. That's the reality, and it's also a fraction of what makes people want to watch this sport.
As others have said: learn to deal with these strikes by either checking them, avoiding them, or putting your fist/foot/knee/etc in your opponent's face.