Beta testers already have an edge on other players due to well, testing the game before it's out. Why shouldn't they be rewarded for their time by being able to keep their names?
I can't really say I see your logic here. You say that Beta testers already get an advantage in that they get to test the game pre-release, which gives them an edge, I agree. But then you say that since they already have an advantage why should they not get even more of an "advantage"? (In this case via keeping their names)
I don't really care that much, but I don't see why they should get more of an advantage than they already have, especially considering what some people already mentioned: getting into beta is not something we get to choose.
Considering the advantage of knowing how things work before non-beta players and the fact that so many want to be a beta tester, I think the beta-playing in itself should be reward enough. Giving them more advantages as a "thank you" for their "hard work" seems a bit over the top to me.
When it comes to this whole first and surname thing, people seem to assume so much. I am not sure if I have missed something, but yes it seems we will have surnames for those who wants (Maybe mandatory.. would be stupid, but as far as I know this is not something they've talked about so we can't know for sure), but who said we could all use the same first name? I have no idea what system they have decided on using, but in the few games I've seen surnames implemented you could still not use the same first name. Surnames were optional and as far as I know not unique, unlike first names which were kept unique.
Why they did it that way, not sure, maybe out of simplicity when it comes to /tell etc, but I would not be surprised if that is the system SE is using for 14 as well. Again, unless I've missed some announcement where they talked about this and explicitly said first names will not be unique because of the use of surnames.