Brunni (./29) :
Oh OK. But then would they effectively refresh the display at 59 Hz ? (i.e. 59 scans from top to bottom) or would they scan at 59.94 and one image would be displayed twice each second?

The former. Fully analog TVs can't duplicate or skip frames by design, because there's simply no memory inside them to store the video. So the electron beam always moves in sync with the input video signal.

(To nitpick, the "no memory" part is not quite true for PAL-encoded signals. As the decoding requires data from both the previous and the current video lines, there's a component inside the TV that provides a copy of the video signal that's delayed by 64 µs, i.e. the duration of one video line. But it's only needed for decoding color, not brightness, and only for PAL. In pure RGB mode, there's no delay in the signal path.)

CosmicR (./30) :
Maybe they wanted to match consumer TV's more closely for the home hardware?

That's my suspicion as well. The farthest away from 59.94 Hz you get, the higher the risk of some TVs not working fine is. Composite/S-Video encoding may be a factor as well ; NTSC is designed explicitly for 59.94 Hz (there's a complicated explanation why 59.94 Hz was chosen instead of the "normal" 60 Hz), so it may cause additional problems that don't exist in arcade machines (since they use RGB internally).