I'm fairly willing to believe that intuitive understandings of "more ethical" will do well for imprecise things like "we'll probably get better results by instantiating a more ethical person as an em than a less ethical one". I'm less convinced the results will be good compared to obvious alternatives like not instantiating anyone as an em.
We see value drift as a result of education, introspection, religious conversion or deconversion, rationality exposure, environment, and societal power. Why would you expect not to see value drift in the face of a radical change in environment, available power, and thinking speed? I'm not concerned about whether or not the value drift is "accountable", I'm concerned that it might be large and not precisely predicted in advance.
Once you entrust the em with large but less than absolute power, how do you plan to keep its power less than absolute? Why do you expect this to be an easier problem than it would be for a non-em AI?
I'm less convinced the results will be good compared to obvious alternatives like not instantiating anyone as an em.
Not building an AI at all is not seen by MIRI as an obvious alternative. That seems an uneven playing field.
Why would you expect not to see value drift in the face of a radical change in environment, available power, and thinking speed?
I don't require the only acceptable level of value drift to be zero, since I am not proposing giving an em absolute power. I am talking about giving human level (or incrementally more) ems human style ...
The reasoning of most of the people on this site and at MIRI is that to prevent an AI taking over the world and killing us all; we must first create an AI that will take over the world but act according to the wishes of humanity; a benevolent god, for want of a better term. I think this line of thinking is both unlikely to work and ultimately cruel to the FAI in question, for the reasons this article explains:
http://hplusmagazine.com/2012/01/16/my-hostility-towards-the-concept-of-friendly-ai/