Interesting post wdigin - and a very valid one.
I do think that my son and my DSCs have had gained a lot from our relationship. Much as it has worn me out, the youngest two DSCs needed stability which they didn't have before. Their Dad and Mum were both out a lot with a series of very short term affairs, they were both arguing a lot with each other, there was a lot of confusion and acting as if they were together but not together. My eldest DSD told me of her mother turning up at the middle of night to drag her out of bed to take her home as she didn't like one of DPs girlfriends at the time.
As much as the eldest ones resented having to be cordial and weren't bought up to have manners, I did bring a sense of boundaries and normality. Their father's house was bare and not homely, I made it cosy and nice. We had Christmas trees for the first time, we celebrated every birthday, I was another rock in their lives.
I was another pair of eyes and ears who would notice if the DSCs needed new shoes, were struggling with their homework. I took the youngest one out every single weekend for five years, to the cinema, to the circus, to get ice creams. I helped take care of them and pick them up. I took them away on holidays without DP.
Actually, if I'd have been able to discpline or even advise eldest DSD she wouldn't have ended up making some terrible life choices that she deeply regrets and both her mother and DP even admit I had a good insight into at the time.
Although for me it wasn't worth it, the stress was nuts. Most of that came from being relegated to the background and the DSCs and EXW acting as if DP was still theirs to control. DP took the easy road as his EXW would put him through hell if he didn't. It wasn't great for my son to have to put up with rudeness while I struggled trying to get his eldest step siblings to treat him with kindness.
I got there in the end, but it was like walking into a new house and having to spend a fortune clearing up and on repairs before you can even paint the walls!