In what way does sending them off by themselves to do 12 hours flying help? That just gives them more opportunity to crash!
Your point might make some sense if you said that they had to make up the 12 hours with dual trainng, but as gasax says, the statistics don't support that.
Are you talking about the USA? In Europe, an instructor can't do this sign off. It needs to be an examiner.
I wasn't aware that there was a 12 hour requirement in the USA.