Without reading a lot of the answers here, I can only say this, having been a kid with step-parents since I was young. (parents divorced and remarried multiple times)...
I think the biggest problem we are really seeing is a lack of forethought and communication between the COUPLE. Now, add in entitled adult children, and you have a huge problem.
If a couple is looking at getting married, any parties with kids NEED to discuss the level of support they are wanting to provide their adult child. I am, of course, not considering some severe circumstance like an unforeseen serious illness here, but in the cases of long term medical or mental health issues-- or unemployment or lack of any drive to be self-sufficient-- this is a conversation people need to have with each other before rushing into getting married.
The relationship I had with mom's third husband -- who became my step-dad when I was eight-- was of course extremely different than the sort of relationship I had with her fourth, who became more or less my step-dad when I was in my late teens. The former disciplined me; he'd signed on to make sure that we kids were housed/fed/clothed. The latter basically told me his house rules and I respected them (we only lived under the same roof for a short time, couple months); we had more of a 'you're my mom's boyfriend' relationship-- I supported myself from 18 years old and onward. I did not expect either he or my mother to support me.
And you are right, S.-- we would tell a mother "really, get your kid grown up already" if a grown adult child were living at home with no sense of participation to the betterment of the household at all. We might even wonder aloud/in print how she raised a kid to be such an entitled, selfish brat, to be expecting mommy and daddy to float them through life.
I try to consider my step-parents as individuals, not just collectively. My stepmom from one of my dads has not been financially supportive, but she's been there at every turn and has been far more emotionally supportive than my own mother-- who didn't have much to offer besides money. So, there's another perspective, I suppose-- that emotional support to grow up into a functioning adult (and she helped with that) is far more important than continuing to pay someone's way through life and not challenging that presumption that they are "owed" just on the merit of being born...
In most relationships, it is the health of the relationship of the parents which determines how functional and healthy the family as a whole are. That still needs to happen, whether or not one of the parties involved is a stepparent. Our spouses will always have their kids from previous relationships, but hopefully HOW those kids are parented and supported will have evolved in age-appropriate ways.