Actually it does make a lot of sense. In the basic form most people suggest this would be incredibly easy to exploit and would destroy any chance at a fair revenge dynamic or war matchup. There is a much more detailed discussion about the drawbacks and many suggestions about how to code around or block those exploits that you can find here:
http://forum.supercell.net/showthrea...n-member-bases
In the end there are two things that kill this idea. First, once all of the potential exploits have been neutralized the code and rules behind it are very complex for a game with such a relatively simple design. Second, the things that neutralize the exploits make the feature so singular in purpose as an occasional training tool that most of the people who request it as an "attack anybody, anywhere, anytime just for fun" don't want it anymore because there are rules, then their interest dies.
As the leader of a clan that embraces new players I would LOVE to see something like this implemented so I and my co-leaders can help educate new players but I also recognize that for the amount of work it would be there are other more important and beneficial things that the developers can be working on like a war strategy UI, slight un-nerfing of hogs, giants/hogs defending themselves against CC troops if they are closer than the next building, deciding if an opt-out button should be available and enforcing it one way or the other, etc.