1) If you forget to backup the map then you can only blame yourself for it
2) Many people dont give credits even for maps the copy anyway. To learn how to make maps you have to experiment yourself or you will never be really good at it
3) An unupdated map can be popular a long time, and even if you update it regularly it wont be popular forever anyway
If you dont protect your maps:
If the map is popular you can be 100% sure that there will be several copies of it floating around on bnet after a short while. And you can also be sure that alot of the people who made those copies will not give you the slightest credit for it, because they copy it to try to make people belive they made it and steal your credits. Ofcourse not all people are like that, but there's alot of jerks and morons on bnet.
But no one can use your map to learn, so if you have found out something special and good in your map then you should share it with others on places like this forum afterwards.
If you do protect your maps:
If you make a popular map you get all the credits and people will remember your name, greet you when they see you, send you mails about how much they like your map etc because they know that you and no one else made the map.
However after you have taken advantage of your map and the concept as much as påossible (til it's not that popular anymore) then you should release it unprotected if you want the concept to live on. Then people can shape the map in new ways and create new maps