The problem is Apple wants the wifi sync to be automatic regardless of what the user actually wants.
Then, to prevent draining your battery with the automatic sync, and to prevent the device losing power while performing a sync, they insist the iOS device is charging before it will auto sync.
And of course, to charge means plugging in a cable, which for me defeats the benefits of wifi sync.
At this moment in time, I have 10 app updates that my Mac has downloaded and I want installed on my iPhone. I'm in the garden with my family, so my options are to go inside and plug my phone into the computer to charge it, negating wifi sync, or download the updates again From the apple servers. I choose the latter in the hope that they one day realise I'm hitting their servers 4 times for the same update because they won't let wifi sync work how it should.