People are free to their own opinion, you won't like it if people do that to you and if you insult people for different opinions and insist that people respect your opinions, than you're a Hippocrate. And angry people tend to angry at eachother.
The rule about treating people the way you want to be treated is actually a universal rule, not just comply to one group.
If you disrespect someone because that person you don't hasn't done anything earn your respect, and that person thinks the same way, then you won't ever get anywhere, and it's just a cycle of angry jerks.
Yes. If they have to resort to name-calling and insults, then that just proves how weak they are and that their argument/viewpoint is completely unsubstantial and flawed. I consider these people inferior and take great pleasure at laughing at them. If you can't state your case in a non-confrontational, equanimous, logical, and polite manner, then I have no respect for you and don't take you seriously.
When someone is calling me names, cussing, mad, etc... I usually stop talking to them. Reason is that such a reaction to just a opinion is a good sign that this person is emotionally attached to their opinions. This may also mean that this is a belief system for them, or a clear sign that they don't know what they are talking about(usually happens when a person allows another person to think for them e.g. sourcing information). So there is no point of trying to convince such a person.
From OP: yes, I think it's totally unnecessary. Like when someone doesn't like a certain singer but another person likes him/her and they say something like "you must be blind if you think that's good", as if their opinion is fact. Its like if they think that so and so is a terrible singer, then he is, and anyone who disagrees doesn't know what they're talking about. You can comment on someone else's opinion with something like "I personally don't think s/he sounds good at all" This way you're saying that you don't share their opinion, but in a non insulting way.
That said, nobody has to agree with another person's opinion. Why should they? Husbands & wives don't always share the same opinion. Parents & children often times have opposite opinions. It's a matter of choice. Opinions are based on many variables, i.e. education, experience, faith, etc. People have a right to feel as they do, even if their position seems antiquated. It isn't an insult. It is merely their opinion.
If the person has good reasons for name calling he/she better have facts to back it up. Normal name calling without first processing the insulting term used in the insulting process too see whether or not the term is suitable for insulting the insult is then not justified. But if for example, the person insulting used the term imbecile on his/her opponent . Let's take a look at the dictionary definition of imbecile:
Informal. A dunce; blockhead; dolt:
Don't stand there like an imbecile. Open the door!.
Psychology. (no longer in technical use; now considered offensive) a person of the second order in a former and discarded classification of mental retardation, above the level of idiocy, having a mental age of seven or eight years and an intelligence quotient of 25 to 50.
Informal. Stupid; silly; absurd.
Usually Offensive. Showing mental feebleness or incapacity.
Archaic. Weak or feeble." -- Dictionary.Com
If the opponent does indeed fit that criteria then the insult should be justified. The person insulting should then go about justifying his/her insult with evidences from his/her arguments.