Totally depends on the situation, and the people.
I'm a step mum, my step daughters are with me and my husband full time, and I think I'm a bloody good one. They are loved, secure and are treated exactly as they would be if they were my own daughters. Their mum isn't in their life now, so clearly the parameters of our relationship have changed, but even when they were only with us at weekends I felt that it was my responsibility to ensure that they always felt wanted, loved and part of MY family, because they were!
The girls step dad is the reason their mum isn't in their life any more, and it's safe to say we would all prefer if he hadn't been in the picture, as his behaviour impacted her behaviour and made it unsafe for them to be with her.
My own step mum is genuinely one of my closest friends, but she is my dads third wife and came into my life when I was 12. I'm now 33, she's 53, and we're really close. I speak with her more than my dad. But she's not a parental replacement, I have my mum. But she's made my dad a better person, my sister wouldn't exist without her, and she's bought so much to my life. My step dad is a grumpy git, but he's hilarious, and he and my mum are like a comedy act. Has no bearing on my life, but is entertaining to watch,
Dads second wife, my first step mum, was a twat, but luckily she didn't last long.
All depends on the people. Clearly we'd all prefer parents to stay together and be happy, but actually, in my case - my parents are better apart with their new partners, and my DH and his Ex-W...? Well, without sounding bitter and slating the ex-W, I'm happier that the girls are with me, let's put it that way (as are social services, and the police).
So, no, I don't agree that step parents are always a negative. I think it's better for children to be in a loving, happy environment, whatever that looks like.