Is it common to see directors acting in their own films?

Speaking as a director-

To step back, one has to consider what the job of directing is. It's a story teller who sees the big picture. Directors have to be able to create the scene and the tone with picture, framing, editing, and of course acting.

Some directors are stronger at pulling the best performance out of an actor; some are better at setting the scene. But inevitably, everything you see they've had their hand in.

Being part of the final product, even for just a split second lets them take the ride they are building. Chris "X-Files" Carter went so far as to have Ten-Thirteen Production's Logo include a voice saying, "I Made This!"

Finally, who better to figure out where to put a little bit of self-homagistic fun into a film than the person directing it.

Not every director will have the desire to do so, and there may be many cases of directors in films that haven't been caught; but it's very common and very likely to continue.