The block is there becaue the plumbers are one step ahead of the siders.
The plumber is handed the block to put his frostfree faucet through, and then later the siders can match up to that.
In the long run, it can be a better looking job, than having the sider try to cut around an existing faucet that may or may not be the right distance out from the wall.
I wouldn't worry about freezing with the block either.
They only seem to freeze when the hose is left attached over the Winter.