I'm not sure if I understand what you mean. I will provide a user case. Let's take the groups I mentioned above: Ubuntu Servers, Ubuntu 10 Servers, Ubuntu 12 Servers.
We use different configurations on Ubuntu 10 Servers and Ubuntu 12 Servers, like software repositories. However, most configurations are suitable for both Ubuntu 10 and Ubuntu 12 Servers.
At the moment I can do two things to accomplish this. Or I create three separate groups, Ubuntu Servers, Ubuntu 10 Servers, Ubuntu 12 Servers, based on regexp. Or I just create two separate groups, Ubuntu 10 Servers, Ubuntu 12 Servers, and bind those two groups to one directive.
It would be nice if I was able to create two groups, Ubuntu 10 Servers, Ubuntu 12 Servers, based on regexp. And then make an Ubuntu Servers group which contains both groups.