I agree with this. I grew up in a blended family and my stepsister still (after pretty much our whole lives being a "family") has never really warmed to my sister and me. I really think our living with her father while growing up is at the root of it and something she's never been able to get past. My stepbrothers, on the other hand, are real brothers to me. Kids can grow up in the same environment and perceive things/react quite differently. It all depends on how the children react to the stepparent. I, frankly, am glad my mother found someone to be happy with. It made my mom a happier person and less likely to take out general frustrations on us kids (she was/is a great mom, though, so this wasn't really a problem). It also helps that my stepdad turned out to be awesome.
That said, it really does depend on the situation. Some kids deal with parents' new partners better than others. I don't think the parent necessarily has to ditch a relationship simply because a child is unhappy with the fact that the parent has found a new partner. Kids need to be taught that everything in life is not always about them, that things will not always go the way they want them to, and that they can't have everything they want. I think those lessons aren't taught the way they used to be. I don't have kids, so I know my opinion probably won't matter, but I was once married to man who already had two kids, and it took his daughter years to come around. She never truly welcomed me into her life as best buds or anything, but she at least learned to tolerate me and discontinue her sometimes disruptive behavior. She confessed that she wanted her dad all to herself. I pretty much told her that that was not realistic, even with parents who stay together. Parents need time to themselves once in a while. My parents always had a date night once a week, and if they had adult parties, us kids were in bed before the craziness began.
I think it also helped that I did try to explain to her that in no uncertain terms would I ever try to be her mother; I just happened to be someone her dad really loved and wanted to be with. Children can be very egocentric, attention-craving beings who hate change, good or bad.