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Because : Once you SubD, you can NOT go back.
Too many figures have been "lost" because the builders SubD too early and could not correct the base any more.

 

 

That's not entirely accurate. I don't know what apps are being used here but if you haven't done any additional polygonal changes after subdividing (ie. adding or removing polygons manually or changing edge flow), ZBrush is more than capable of reverting your mesh back to its starting point.  That assumes the OP has ZBrush.