First of all this is just my own opinion. Data wipe when official launching is more fair to others because at first we do not know how long more for the game will be official launch. Those people with no invited in Closed beta will feeling very unfair because it may wait for few days or weeks but the progression of the CBT tester are a lot further than the new player.
If as JeForceX mention that ranking is not important and its just the game, then what is the point of setting the ranking in the game and most of the game that top ranking are giving reward, those CBT tester have a lot advantages than the new player because they are more understanding the game and their hero are more than the new player.
As Ryozanki mention that new player able to catch up the CBT tester, I believe that it may is the truth. But in this point please take note, new player need to CATCH up and not PLAYING together. In order to catch up, new player need to spend more money and time than the CBT tester. Although that level may not really important in the game because in facebook got mention that the strategy of the game is more important.
So the new player firstly is lower level of hero then lack of experience or knowledge of the game. From my point of view that I can't see any balance or fair to the new player. For example, when I just login the game when official launch then look at the ranking that there are level 20+. If data wipe make unfair to CBT tester, is only the progression of the game and the time they spent is to get gain the experience and the knowledge of the game rather than we are unable to get into CBT. I think that those player are invited to CBT is volunteer to test the game because the main point of CBT is the testing the bug and not taking advantage of game progression.
Every point just point of my view. I am happy with anything that the game can official launch early