Why Translucent Mirror Can Not Lift Up?

Given all the issues I read in this forum,

1. ghosting issue

http://forums.dpreview.com/forums/read.asp?forum=1037&message=36125668

2. dirt and dim issue

http://forums.dpreview.com/forums/read.asp?forum=1037&message=36134258

I am just wondering why two modes:

1. one with mirror up when continuous AF is not needed, like traditional DSLR (single still shots), i.e., mirror is in the position for AF and then up for taking picture.

2. and another with mirror in the position when continuous AF is needed (movie and high speed shots)

was not possible in Sony SLT design.