Hi Fred,
Well chumming can cover a wide territory. I used to live in Kailua-Kona, Hawaii and there is a neat little protected bay called Magic Sands. We used to have to get after the tourists for chumming so to speak as they were feeding the reef fish frozen peas. What's the big deal...? The fish fill up on food that has no nutritional value to them, kind of like being fat but starving to death.
On the same island town there was an Atlantis submarie operation that engaged divers to chum for fish as the route was not really populated with a lot of marine life. They quit that practice after some divers were bit by morays who had become used to the regular feeding and aggresive when they didn't get fed. We used to sing a song dedicated to those divers (sung to the tune of When the moon hits your eye)... Stick-a your hand in a crack, if you hand don't-a come back, that's amore (a moray).... The divers were being just plain stupid.
So there's two reasons for "no" that I have experienced. My personal thought is that it may just tend to unbalance the natural order of the reef. I am largely undecided about spearing lionfish and trying to get sharks, eels, groupers, etc to eat them. Lionfish are not in the natural order of Caribbean reefs so don't know what the right thing is on this issue in the long run.
I have done a shark dive off Roatan and it was OK, but not sure I'd do it again. It was up close and personal with the sharks but if I had my druthers, there are lots of places to dive with sharks without feeding them. If I want to dive with sharks now, I'd do Cocos Island or the Socorros off Baja.
I was on one dive off Roatan out of Coco View where the DM was trying to draw a fish out of a small rock cave and he crushed up some urchins. I thought that was pretty stupid and made mention of it. I guess I was the only one offended. But then, besides my dive travel business, I am also president of a non-profit marine conservation group that focuses on sea turtles. So I guess there's my third reason "no" to chumming. Why intentionally kill another critter just so a couple of divers can see a fish they might have otherwise missed....?
As to the touching of the critters and reef....I must admit I stick my finger in the sand to stop while taking pics. I also do the same on the occasional rock. Having a saltwater aquarium and breeding corals, I am well aware of the difference between a rock and live coral. I have also touched the occasional friendly grouper that brushes against divers in the Caymans. Last trip off Little Cayman, we literally had to gently push a grouper away from us as the fish would swim right under us. Given the way I have seen groupers dive into rocks after prey and turtles scrape their shells on rock overhangs, I'm reasonably sure a light human finger touch doesn't endangers the critters. That said, I never dive with gloves regardless of location. Using the Cayman manta trips and other areas where I have come in contact with rays, it seems human touch is not fatal to some marine critters.
I have also been fascinated by some cleaner shrimp and have found with patience the little critters will climb on your hand. Kind of an interesting interaction with the ocean, but not really chumming unless you consider the shrimp picking at the skin on my hands....
That's my two cents....