As far as why bloodline limits regency, I think it is so you don't have guys with low bloodlines with large domains. Since you don't have as much "God Juice" you are not as fit to rule. Your neighbors will sense this and you won't have the RP to handle all of the contest actions and your domain will be reduced to something that you can manage. It is the reason that even though you are a great mayor of your town of 5000 you have no idea what it would take to be President. It also prevents one from quickly raising their bloodline since they won't have tons of RP to burn.
With commoners becoming blooded, I like to always have the option of making characters blooded later. I know the rules do not allow for unblooded bloodtheft but I don't have a problem with Rule 0ing that stuff. :P

CW