Most of the really good spearos I know (including a few of my occasional buddies) generally spearfish while freediving only. Not only do they prefer the challenge, but they know that the fish they seek (such as white sea bass and yellowtail) are much easier to approach that way. Although I also know a few excellent spearos who fish on SCUBA, many of the others who do so are new to the sport having just purchased their first spear or gun and "want to kill something today." Yes, I've actually heard that cry... not once, but several times. They give the good spearos a bad name.
I heartily agree that anyone who gets their fish wrapped in saran from the grocery store generally have no clue as to the ecological devastation caused to obtain those fish... it is too "abstract" from their daily lives, just as nature is largely "abstract" to a city dweller... unless they actively get out to camp, hunt, etc.
Personally, I think everyone should have to hunt and kill their own food at least once in their lives. Back in the early 70's we had our students fish and hunt for food. If they were vegetarians, we had them strangle corn and beans and eat it... er, I mean grow their own in our organic garden. Although there are good ecological reasons for vegetarianism, I've never understood why some don't see the linkage between eating seeds and abortion.
I should add that although I used to spearfish in the 60's and 70's, I stopped doing so in 1975. In fact I stopped all hunting and fishing then. I used to hunt quite a bit for food and ecological control of non-native feral animals. However, I'll eat what my buddies catch and I might even pick up the gear again myself with the way food prices are rising! I'd just have to figure out how to hold the video camera in one hand, the spear in the other and still be able to control my inflator!