Step families can be positive things and in some cases more positive than parents who stay in an unhappy relationship.
My DSD has always seen it as a positive thing. She has 4 half siblings, 2 sets of grandparents, two sets of step-grandparents and a seemingly endless number of aunts, uncles and cousins all of which come together to form a happy and supportive family. When she was younger, she also appreciated having a whole ton of stuff at Christmas from all parts of the family
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So what would have been better? Her mother staying partnerless and unmarried and her dad staying partnerless and unmarried. Just her moving between those two households? I am sure she'd disagree.
As for the step-parents who don't like their step-children, well in my experience the worse of those cases is where you have a disney dad who doesn't set any boundaries or a mother who makes it clear she has problems with her ex and his family. This leads to unruly or bad mannered children who it can be really trying to deal with.
Also, there are no end of threads on here about not liking children that aren't your own, so there's no magic wand that you wave as a person where you suddenly like your partners children. In my case I had been with my partner for 8 months when I met his daughter. If she had been rude or we'd not got on well, it would have been bloody hard just to give up loving someone just becasue of his daughter. Even liking her didn't mean that things always ran smoothly with DH acting like an arse on several occasions, but thats another story.
The step parents who post on MN are often doing it so they have somewhere anonymous where they can come and vent or get advice or try to make things better. I know that I have posted on there on numerous occasions when I've needed help or I have been pissed off beyond belief. However my DSD or my DH would never know as by having somewhere to vent and get it out of my system, I was able to carry on being a reasonable human being in RL.
Well, this has turned into a bit of an essay so in summary - step / blended families can be a good thing. Staying single isn't always better than a new relationship for children and stop bashing the steps who post on here as until you walk a mile in their shoes you don't know what its like. I think that about covers it!!