Wow, this thread exploded while I was at work!
I'm not a believer that royals should only marry royals although I wouldn't be opposed to it.
But the middle class is so large, it covers a diversity of backgrounds and outlooks, some which may prepare one better for a royal life than others. Princess Mary of Denmark comes to mind. Her family was not wealthy, her father was a college professor and she had a background in PR. Careers at the University and the social strata among the professors as well as the unusual practice of tenure, gives University life a lot of parallels with the lives of the royals even though professors don't make much money at all. Professors have to mingle with all sorts of society - high and low - to make it in a University and they have to master the social nuances of each social class for the type of events they attend. It is a very social and very tradition-bound career much like being a royal although it doesn't make much money. Mary, herself, had a background in PR. So I think that the combination of Mary's background in PR combined with her growing up in a University professor's family gave her an insight into the challenges she would face as a royal princess.
I thought that Kate and William had a lot in common at the University however, that may have been due to the fact that University life for students tends to minimize class and culture divides in favor of a single unified University esprit de corps much like the military tends to minimize cultural and class divides among its fighting men.
However, when you take people outside of the cocoon of the University, other influences start to take force, such as people's own background. From the dot.com millionaires I know, there is a big cultural divide between them and even the more traditional banker/lawyer/doctor type middle class success story.
If you think of the Royal Family as a 1000 year old brand image, then its natural to look for signs if the 'image' that a newcomer projects will complement or detract from the core Royal brand. When Kate was at the University and even when she was apparently doing nothing while the press was complaining about her not working, I still thought her image consistent with the basic values of the British Royal Family brand.
However, these latest PR attempts don't even seem like they're marketing the same thing that the Royal Family is trying to project. This gives me the idea that Kate and her family may have a totally different idea of how to market themselves than the Royal Family does and this disconnect for someone who is marrying into a very high profile position can be problematic.
Now if she had gotten a job out of the University at an art museum, even if it were filing papers in a back room, that would be more consistent with the overall Royal Family brand image.
Of course they have to love each other but for such a high profile position, I don't think love is enough. Andrew and Sarah loved each other a great deal but she still had tremendous problems with the Royal Family because even though they all loved her privately, her image detracted from the Royal brand.
Yes, you can see that I've been working with marketing lately.
I have marketing on the brain but I think there are great parallels when you think of the Royal Family as a family business with a brand image that they want to maintain and then think of who or what could most reasonably enhance it. It doesn't solve the love problem, of course, like I said, the two have to love each other, but the marketing question is important too and I think it shouldn't be overlooked just because two people love each other.
edited to add:
I think what bothers me about the dot.com connection is that you don't have to be social to make a lot of money in the dot.com business and the position of a Royal Princess is totally social. That's the big disconnect for me, and I think it may be hard to overcome.