Strong players almost never play this way. and
are diagonal contact plays. One such play can be justified if you then attack the opponent's heavy group.
In this case Black is doing well, since affects both White's group on the left, and
.
Here, on the other hand, it seems that Black is only interested in immediate territory. Since both a and b are weak points of Black's formation, there isn't much real, secure territory so far. White's stones on the outside have gained greatly in influence, though.
This way of playing by Black contradicts the initial formation's basic nature: stones on the fourth line should be a source of power.