The only problem with this is when travelling downstream where there is not enough room for a narrowboat to swing around, you definitely don't want to end up wedged across the stream.
Sometimes common sense must contradict the 'rule'. Hopefully we'll never need to have to use ones anchor, however I grew up on the Great Ouse and conditions can change rapidly catching even experienced locals out.
I live in West-Wales and just a couple of weeks ago we suffered tremendous rainfall causing our local river systems to rise by 20' practically overnight, unfortunately claiming a young mans life and breaching flood defences causing millions of pounds worth of damage. Always prepare for the unexpected.