There's obviously a difference between a school that becomes an academy by force and one that becomes one through a MAT. Certainly there's some element of choice with a MAT, but it very much depends on the composition of the MAT, how many schools are included, primary, secondary, good, outstanding, RI ETC.
I think that the HT role will be reduced, as they won't have autonomy over their own school and there will be an executive head of the group. The HTs in the MAT will all have responsibility for all the schools, so if one school is doing well and another not, the responsibility and subsequent actions will happen to all. You also have to consider whether the MAT will be a group of equals, or whether one HT will deem him or herself in charge.
That, together with the comments regarding public buildings, pay and conditions, parent governors etc. In fact, it may well be that governing bodies will be dissolved and the reconstitution will be based on the recommendations of someone who has little or no knowledge of individual schools in the group.
Whatever is the situation regarding the government, in a few years time there may well be a change and a change to policy. Yes, Labour supported it when they were in power, but many things are different now.
The accountability lies with the MAT, but it will be interesting to see how that works in practice. We already have headlines about finances, large salaries for execs, poor results. Who knows where this will take the education system.