why the use of MV-file can degrade quality?

Why the use of MV-file can degrade quality?

It reduces quality because as the rate control is refined during multipass the video is encoded slightly differently during each pass. The MV re-use enforces fixed motion vectors, where in fact the optimal motion vectors may change very slightly during multipass because the video is never encoded the same way twice. The result is that your video will still look good, it just won't look as good as possible. MV re-use isn't so important in 5.1.1 because the encoder is already so fast anyway.

Ok, tnx for the info ;)