Why do the OFDM training (and payload) symbols have silent sub-carriers (aka virtual carriers)?
From the first to the sixth OFDM-Symbol you have spread virtual carriers equally across [0,..,51] with a distance of 3. With this way you have spread your 'energy' loss over a wide spectrum. In difference to having a single carrier serving as virtual carrier all the time you can transmit information over every 'data' carrier and maybe have a constant mean energy level across all subcarriers.

You could use this virtual carries to detect carrier frequency offset (CFO) since you know the pattern. In the worst case you have to store 6 OFDM-Symbols to find the beginning of your pattern. 

If you have an OFDM-Symbol with 64 subcarriers you can set 6 carriers at each end as subcarrier and use the other 52 with this virtual carrier pattern to transmit information with equalization.

I think this virtual carrier pattern is not connected to the Schmidl &amp; Cox synchronsiation pattern.