This is the one thing that I felt clash of clans needed.
I don't see any issue with this. One of my biggest issues, is the lack of ability to practise without losing tons of resources. For me I enjoy spending time practising and improving my techniques.
I think they need to implement maybe the ability to attack your own base however. That would be more beneficial for personal improvement. However I can see that deterring players from actually using the clan to help each other.

