NPR had several interesting discussions regarding the controversy over girl-themed Legos, and gender in toys [2][3].
1: http://www.npr.org/templates/story/story.php?storyId=1457051...
2: http://www.npr.org/templates/story/story.php?storyId=1603654...
3: http://www.npr.org/2012/01/18/145397007/gender-controversy-s...
On reflection though, I think anything that helps get girls interested in the concepts is fantastic. This simply wouldn't be of value to my own daughter (or my nieces for that matter), but I can see how it would be really helpful in getting girls like your niece interested, and that can only be a good thing.
However, I feel like parents who are inclined to buy this might not be likely to have princess-obsessed kids. But I am not a parent, so I don't know.
Changing the interaction should come second. Dumbing it down is almost never the solution, yet it's often the first solution people turn to when trying to deal with a gender gap. It's as if they think the issues will go away if they speak very slowly and loudly.
(I admire the objective, but I believe I would like the project better if it said "an engineering toy" instead of "the engineering toy." The implication differs.)
Why yes, yes they do: http://friends.lego.com/en-us/Default.aspx
They're controversial for the same reasons people are complaining on this thread, but they're selling. (See http://abcnews.go.com/blogs/business/2012/09/lego-friends-tr...)
Users care about things like color far more than engineers often suspect. If changing the color makes a substantive improvement to outcomes you care about - change the color.
The story + building is the innovation - meeting girls where they are and presenting construction/engineering themes through something other than a jumble of Legos. (This coming from a girl who loved Legos but didn't end up as an engineer)
Really fun to build.