I'm a new cacher ... though I have taken the boys letterboxing previously - but only the 'drive up to a Dartmoor beauty spot and see how many we can find' type ...
We have found 3 geocaches here in Sweden where we are now living, and hidden one which should be online soon.
The reason I decided to hide one with the boys was as part of explaining to them what the boxes are, how people have to site them carefully and check on them, look after them, write the co-ordinates and clues ... etc. In other words to build their (OK the 6 year old's) respect of what goes into placing a cache so that he appreciates what both sides of the game are like.
I have planned a series of three letterbox caches, having bought three related ink stamps, and these will be at local but little known beauty spots near here that are favourites of our family. I am not personally prepared to take on the responsibility of caring for more than that!
I think if I had more finds I would be absolutely in agreement with (many of) you about having a minimum number of finds before being able to place a cache ... but under the current rules it has served me very well to introduce both sides of the game to my boys (6 and 2.5) at the same time.
Hope that helps explain why some newbies are posting caches almost immediately!
Helen