I agree they are vastly more dangerous because they are illegal. But you know, alcohol is legal, and people still binge drink, fall in ditches, have random unprotected sex, get pancreatitis etc... and it isn't even addictive. I'm just trying to say that even if heroin were legal, it would still be unacceptably dangerous, because at the end of the day it's a highly addictive general anaesthetic. Even with 'vastly less' problems, which is your conjecture, there are still significant problems. Anyway, we both clearly think that harm minimisation is a good policy.
Yes, that's right, if someone is in pain, the respiratory depression is less. This is evident to anyone that prescribes opioids regularly or looks after people with chronic and acute pain (as I do). It is also evident that the ever escalating doses and withdrawal symptoms demonstrated by recreational users of heroin for example, are not manifested in people with pain. So that's why I use the word 'fundamental difference', because the coupling between dosage, physiological response and behavior is completely different.
Lots of people don't enjoy taking opioids, they get nausea, nightmares, constipation etc, or it just doesn't work for their pain. I know this because that is what is reported in controlled trials, and because people complain to me about these side effects all the time.
Reference:
http://www.ncbi.nlm.nih.gov/pubmed/8867254
More general overview:
http://bja.oxfordjournals.org/content/100/6/747.full
(Glad you are truly interested, but in the time it took to write your sarcastic comment, you could have just googled it, and we'd both be better off right now.)