Well, i can only see advantages on this, for gamers and for the Obsidian team.
1- The user imput
We all now that one of the major problems with obsidian games are the horrible bugs their games have (its true, sorry mates haha).
Same with some mecanics that are not fully tested because of either lack of time or the tester team wasn't really good (i heard somewhere that it wasn't obsidian who did the testing for KOTOR, not sure if true).
If all the backers have access to the beta i believe the game we bough could be so much better...
2- Much more early access to the game = more backers
Many people don't like to give 100 euros for a game and have to wait 2 years to play it.
Ok, we love you guys and we trust you but if we got early access we would be much more happy!
It would be also a appealing thing to new backers who are insecure either to buy the game or not.
If they have to pay 140 just to see if they really want it, i doubt any sane person would do it.
Now 20$... That's much more easy.
I'm sure there are many more advantages other people would like to suggest, right guys? haha
now, the cons...
I can only see history spoiler as a con.
But, we don't have to play the beta, its a player choice, so it isn't really a con.