@RoJeC, sorry my explanation is not clear.
It seems to me that the basic function of the SYNC button is to match the BPM of the opposite deck. The fact that the button also causes the deck to start playing is what I am questioning. If the deck is exposed to output via the fader, the audience may begin to hear your mix before you intended when all you were trying to do is quickly match BPMs to set up for performing your mix.
If the intention of the button is to match BPM and play the track at the same time, then to your point about "basic skill", yes, the DJ could be mindful. But there are already 2 other buttons to start play (Cue & Play).... 3 if you include the sample button(s). So why should the Sync button also autostart? Its not a big deal I suppose since you could categorize it as work-flow.
Its just a suggestion.