Interesting debate.
I think the real game changer here is Facebook. I have to admit that if I adopted again and the child's name wasn't in the Sam/Lily category of ordinariness, I would seriously consider a first name change. Not because I am against contact - far from it - but because I would want to protect my children from contact not made at a time of their choosing, and without a chance to prepare. I think contact should be led by the adopted person, and social media is a threat to that.
And of course in a minority of cases the birth family will pose a significant risk to child or adoptive family if they traced them. Not that usually the case, but if it is then I think the name should be changed.
Facebook aside I think the pros and cons are:
If you adopt any child older than a tiny baby they ARE that name - however daft it may sound. By the end of intros you may never be able to imagine the child called anything but Ebeneezer or whatever. They will make it their own.
A child being adopted has just lost every anchor they ever knew in their small world. To my mind it's not the moment to rock their sense of identity still further by calling them a different name.
Adult adoptees who've had names changed have spoken of feeling as if their identity had been remade by their new parents. I have to admit that on reading the book of the film 'philomena' that really struck me. The boys adoptive parents evidently just decided to call a three year old something different. You can't wipe out history with a child of that age and I think it is often felt that name changing suggests you think you can.
That said other adult adoptees have expressed profound relief that their parents got rid of a dreadful name on their behalf, or have been very glad to lose a reminder of a traumatic past history, so I think it does all depend.
On the other hand - there is an important aspect of claiming the child as part of their adoptive family that it is unwise to ignore. I would worry that some children might feel their adoptive parents didn't care enough to choose a name for them.
I think often the best way is to choose middle names and keep first. My DC know that some of their names came from birth parents and some were chosen by us. They have both experimented with going by the names we chose and if that is their wish in adult life, no problem.
There may be other reasons to do one thing or the other - perhaps if you adopt transracially and some/all of the child's names reflect their culture of origin, it might be very desirable to keep them. Or if a name is a link with a family member it might be either desirable or undesirable to retain it depending. I will admit to losing a middle name which was a link to a very negative figure in one DCs life.
And finally, whoever worries about unusual names should have a good look at what the middle classes call their birth children. My DCs mildly unusual names are NOTHING compared to some of the things their little friends go by! 