We had a really interesting thread on transracial adoption a while back with lots of views I'll try and find/bump it for you.
My view could generally be summed as 'not all children would cope well with having adoptive parents of a different ethnicity, and equally not all adoptive parents are suited to raising a child of a different ethnicity to themselves'
Therefore careful assessment is needed of each potential parent to see whether they are aware of issues which may come up and how they might handle them. The world is not colourblind and it won't help to pretend that it is and just accept any parents who say they would adopt a child of a different ethnicity. Similar to a potential parent who says they would adopt a child with say, moderate attachment issues - that parent might be very well suited indeed to such a child but it's imperative to discuss the issue thoroughly with them, to make sure they have their eyes wide open and they are prepared and that this is really what they want and could cope with. Because there will be extra issues in their lives if they go that route.
On the child's end, we also need to remember that they will lose certain things if they are adopted by parents who look very different to them. Privacy for example. The whole world would be thinking 'adoption' as soon as they saw child with parents, and suddenly this child doesn't have the luxury other adoptees have, of being able to choose who to tell about their adoption. They will get constant questions and comments they can't avoid in all likelihood.
I'm not against transracial adoption, but I'm cautious, and think that ethnicity does matter. I find it disappointing that the debate in this issue is dominated by two loud and opposing voices, one saying transracial adoption is wrong and cruel, and the other saying that colour doesn't matter at all in the 21st century and all the kids will be fine whatever race their parents are. Because IMHO neither of those are true, and its more complex than that.
As to whether it would ever be so important that a child should stay in care, I think in a certain situation with a certain child it could be that important yes. For other children it wouldn't be. Consider a child who specifically says they only want to live with parents who look like them for instance. Or a black child with low self esteem who thinks being black is bad and they wish they would be white instead. That's a real example by the way, from an (white) American adoptive mother who adopted a black daughter. She posted online after her daughter (aged somewhere between 5 and 8 iirc), who was very upset, told her she wished she was white. Because this little girl had been abused/neglected by her black birth parents and subsequently fostered and adopted by several white couples in pretty white neighbourhoods. You can see why the poor girl came to think that black people (like herself) aren't nice people and why she hated her own skin colour.
Anyway I've rambled enough now, I'm going to try and find that thread.