My take on 'thread hijacking' is that the technical reasons, like list managers, email clients, technical standards, etc is a secondary reason for it being a bad idea. I think it is just plain bad manners. (violating social standards) A thread is like a conversation and if you reply to a message with content that is not relevant to that message, you are chipping in.
For example, say I was talking to a person(s) about email clients. One person would say: "I like to use Evolution, because it can show threaded messages" Then I reply to him: "The weather here in SA is getting colder now"
Some people may say that I am rude, others may think that I am slightly crazy, because I replied with a comment that is totally irrelevant to the discussed topic.
So, if we see 'hijacking' a conversation as rude and improper, then how can we justify 'hijacking' a thread? It comes down to the same thing.
When you want to talk about something else, start a new conversation. Same with email.
I can tell you why some people "hijack" threads. And it boils down to not being aware of what they are doing to people who are using threaded email clients. Some people will simply reply to a mail so they don't have to worry about knowing the email address to post a message to a list. They change the subject and throw in the text according to what they are writing about. They don't KNOW what this does to a "thread". Now the only purpose this thread has served was to educate people on the ramifications of replying to a thread instead of creating a new one. I don't think people are neccessarily being rude, more like uninformed. Having said all that, this thread has now outlived its usefullness and I would suggest those of you who wish to argue amongst yourselves, maybe private email would be the way to go?