I think these terms are thrown out as derogatory when you're a bit wound up about something but then if you're asked to sit down and consider it, eg on a specific thread, you're not likely to be bitterly spitting out venom. I can do bitter and venom on demand though... lucky you eh? I don't do so well on making sense but here's a start.
I think of myself as working class with a middle class education. Being born to working class but surrounded by middle class who rarely had to struggle for anything growing up I think of the classes as different worlds. Different worlds of expectation and opportunity, based on wealth.
Coming across adults who think they are hard-done-by if they have to choose between having their third foreign holiday in a year or having their bathrooms done up, or going on about how they're worried about their poor 20-something kids living at home who won't take jobs or sign on because it's beneath them brings this into sharp relief. In working class circles you struggle to pay the bills, put food on tables, and kids have to be independent adults at 18 at the latest. Anything extra is a big struggle. The kids raised in these different circles have completely different ideas of what makes a normal life, and so it goes round and round and round. Not to say nothing bad can happen to middle class people obviously, but working class people are more subject to social ills. Violence, abuse, alcohol, etc.
Then add the point that middle classes are not inherently more deserving of their wealth and status. It has been shown in studies time and again that most wealth is inherited not earned. There has been some social mobility so some boundaries are obscured, but not as much as you probably think (and it is reducing). Higher paid jobs are still largely occupied by those who come from higher-paid parental backgrounds and better education, i.e. middle class. People who work in lower-paid jobs are not inherently less deserving, they simply haven't had the same opportunities. In any case we need someone to do those lower-paid jobs so why load them with status.
So on the whole middle class is an insult from me. Means the object is over-privileged and has no idea what real life is like, at the expense of the poor.