Compromised diagonals and joseki 1
Without a stone at one of the marked points, this is certainly not joseki.
Given a black stone at (or at the marked point), the exchange / is some improvement for Black: it possibly makes White heavier. Black now at a leaves White's stones floating.
White's other natural resource is at here. There is a ladder question involved. In pro games may only be played when the ladder is good for Black: for example if White invades late and Black has some central influence.
BobMcGuigan I don't think I'd want to play in the first diagram above without some support to the right or, perhaps in a Chinese Opening type of situation, but I wondered how bad this move could be even if the ladders favor white.
For example, if things went as in the next diagram, it is clearly a bad result for Black since the marked stone is in the wrong place (it should be at a or b).
Andre Engels: I assume this is more or less equal, given that -- is a joseki, (although both a and b are clearly more common than ). The joseki is discussed in 3-4 point high approach, double contact, wall