Well apparently you are not taking into acount what makes diving appealing for different people. Some people prefer to dive caves, some prefer open water, some prefer spearfishing, some people prefer to carry a hand spear. It takes all kind in the sport of diving, and to say that all people should simply take pelasure from diving, for the sake of diving is much to limited a line of thinking.
To say 'don't do it', without first qualifying that statement, and without taking into acount how others feel about it, is a tad selfish. This was said and done by a person of admitted low skill level. So you're not basing your opinions on what you've experienced, only on your thoughts on the matter. Obviously you know others will feel equally strongly about the opposite opinion, since you expect flames.
So let's see what you've based your opinion on:
"Don't do it. It's not necessary to enjoy diving. "
For you. Others may take great pride in their skill level of hunting and stalking prey, that takes skill, far more than simply dangling bait over the side and hoping a fish takes it. As I've stated previously, certain divers prefer certain extremes in diving to enjoy their type of diving. Spearfishing falls into that.
"Where do you keep the speargun - does it dangle off a wing/BC? If so, how close can your buddy get to you in an emergency ie OOA without a) Puncturing himself or you 2) Takes more time to get to you 3) Could puncture wing/BC and/or drysuit/wetsuit."
Well, most divers I know who spearfish, do so with other spearfishers, and they know how to handle an OOA situation. They also carry their speargun on a tether so it can dangle from them if they need to drop it, or they will simly drop it. Locator beacons are cheaper and cheaper by the day. As for fear of puncturing vital life support gear, their is inherrent danger in all that we do, especially diving. To not do something simply for fear of the harm it may cause, no matter how remote is to cheat ourselves. But we also must continually train if we are in an environ we may face peril. Trainin is the key there, NOT choosing to not face the peril.
" 4) Probably cares more about the prey than his buddy 5) May like to dive solo - (heh for those of you who like solo diving that's your choice - I'd probably die at my current skill level so I would not do it)"
Cares more abou his prey than his buddy... If both divers are hunting with an experienced speargunner, they have the same common goals, and are familiar with the limitations they've imposed on themselves by spearhunting. They know the distance they should keep from their buddy, and they know that their buddy may have a lessened sense of their immediate surroundings. Many spearfishers will have their buddy be their spotter which means they spot the prey the hunter cannot see, but they also keep the hunter situationally aware. They also take turns at this, so all is fair for both divers/hunters. As for solo diving, again thats a choice, and you've admittedly "not enough skill" to do it, so please do not knock it if you can't or havent done it.
"Secondly, possibility of attracting unwanted sealife ie largish sharks which increases the probability of an incident and adds stress to the dive."
We can attract a shark strictly from the way we look in our wetsuits. We can be shark bait simply by being in the water at an inopportune time. We can fall ill or die for any multitude of reasons, again, refer to my comments on fearing things and cheating ourselves.
"Thirdly, fish are precious enough and killing 'em underwater with a speargun just isn't cricket - why not go fishing instead where it becomes a fairer game."
Some fish are precious. Some are quite common. See my above comments about dangling a line versus stalking/ hunting the fish of your choice. I think it takes greater skill to hunt an stalk a fish, without scaring him off and triggering on it's natural self preservation instincts, than it is to take advantage of their feeding instinct, and bait them to take a hook in their mouth. Take advantage of an instinct, or try to evade an instinct, you tell me, which is more sporting?
"Finally, if a speargun misses from an inexperienced bloke, it could cause damage to a reef/coral or other endangered sealife."
Spears tend to loose velocity quite quickly after a certain distance has been traversed. This 'reason' for not spearfishing is kind of weak. ALL divers should take care of the environments they dive in, spearfishing takes no exception to this. Again, most spearfishers I know do not hunt in coral-reef areas, preferring more open water. And are selective of the backdrop of their target should they miss.
Hopefully, you'll see things differently, or at leat be more open minded after you've gained more experience. But only time will tell.