As the themes of "risk" and "consequence" are concepts that I always try to keep in mind when running campaigns, events such as dismemberment or "scarring" (physical, mental or emotional) are options that I like to keep open.
Ultimately, it very heavily depends upon the genre, play style, themes and goals of the campaign. Some genres lend themselves more readily to such permanent changes. Take a dystopian future, street-running type of game like Cyberpunk 2020. It is virtually expected that people get hurt if they get into a fight and that hurt can often lead to irreversible impairments, much like in real life. This is because a game like Cyberpunk tries to keep the brutality and serious nature of combat intact. Have your hand run over by a car and be prepared for some bad news when the doctor starts talking about amputation.
But there are other games that have a different "tone" to the action and that emphasis a different level of punishment and recovery. In a super hero campaign, a character might have his legs pinned beneath a 20-ton dump truck and then be walking around just fine the very next scene. It's about the context and the expectations of the game.
In a general sense though, such permanent disfigurations can make for a very dramatic statement and one that can leave your players with amazing stories to tell...if done right. It should usually be very rare that it happens and often only in a moment of very climactic and oppressive action:
"You burst into the chamber to rescue the divine scepter from the clutches of vile Nazarakt's fortress when you are taken by surprise. Standing beside a gruesome altar with fresh offerings to the dark gods, Nazarakt himself waits patiently, as if expecting your arrival and the scepter is nowhere in sight!..."Foolish mortals! I knew of your plan the moment you left the city walls! Your own king did not even realize that he has been in my service since the beginning! And now that you are here, my curse can finally be undone!"...Nazarakt lifts a single, decrepit finger at you and a stream of hellfire boils around you. You feel the flesh on your neck and face warp and crack as you fall to your knees screaming and struggling to extinguish the agonizing flames."
Maybe a little melodramatic, sure, but this will likely be a moment never to forget and you can bet that if the evil sorcerer somehow escapes death this time, the character (and player ) scarred by his devilish magic will carry the grudge to his grave.
Villains are more memorable and exciting if they can actually threaten the players in some way besides simple *death* or *kidnapping a relative* or some such. But if a villain's actions kill the hero's loved and trusty steed...you have just laid the foundation for many sessions worth of gripping, player-driven role playing fun!
Like all tools, it should not be overdone. But used sparingly and dramatically, it can have an impact on your game that you can't get by simplying rolling 10d6 damage.
HARRY DRESDEN — WIZARD
Lost items found. Paranormal Investigations.
Consulting. Advice. Reasonable Rates.
No Love Potions, Endless Purses, or Other Entertainment.