If Wiccan is based off of old Witchcraft and most witches were constantly hiding their ways, wouldn't you think that having a book of spells would be instant prosecution? In actuality, Book of Shadows or Grimoires are not very old.
Even by 1750, less than 50% of women could even sign their own names, much less write in lengthy books and men were only a little above that statistically. Very few had a decent education and even fewer of those with a decent education were practicing witchcraft.
And as Brysing has pointed out many times, witchcraft was illegal in England until the middle of the 20th century. Someone else recently pointed out that in many places around the world, it is still very illegal. So writing anything down was very risky unless you were weathy or higher class, in which case it was generally overlooked due to your higher importance in society, unless you stepped on the wrong people's toes in the process. Even the famous occult writers went through hell due to their literature.
I think its wonderful to write things down and I do it all the time. But I think that it is important to understand that such books for personal use, seen as a staple in the craft, is a misconception of modern society.
So all these little rules like "show no one" was probably the fear of prosecution to those lucky to be literate enough to make one.