Conceptually I think the biggest difference is that a SB/SD that starts from a non provider does not start with the girl thinking, how much do I need to make this work. A provider starts with how much do I need to make this work.
If you don't like being manipulated you have a far less chance of being manipulated by by someone that is younger, not in this world and not looking at the almighty dollar as the driving factor. Maybe like pleasantsurprise they enjoy older men, the have a taste for things they can't afford etc.... so it appeals to them.
In college I dated many beautiful women and part of allure was what I was going to be in life for some. So they looked at me as person with good potential. I didn't make shit for money. Now take a young lady that might be attracted to an older more sophisticated guy that has taken care of himself. and there is no thought of how much is this worth to me to this. It simply is a cool thing, that then starts turning into a financial relationship, because the guy wants to help her and enjoys someone that appreciates even simple things. Then they start spending more money because they want to. The thought of how I work this type of arrangement around my real life and what complications could arise worry me to a degree.
Now take a provider who starts out with what is in it for me financially? This is where they start. So from the beginning it feels less organic, more of a business deal. Maybe as some have stated a hooker on retainer. This type of situation it less likely to have any kind of emotional element that is part of the other scenario. but it could develop I guess.
Many fail to address the emotional element of these type of relationships as part of the process and unless you do that it is hard for a guy to see how a provider fits in that equation. Lets face it is almost the opposite for most guys who hobby, they want to see a provider to remove the emotional aspect of sex and closeness and go back their real life.