Putting aside for a moment, the restrictiveness of having only 10 potential segments and having to define them before and experiment has started, it would be exceptionally helpfull to be able to combine multiple segments.
Visitors who have seen a certain page, using a particular browser.
Visitors who have coverted 1 or more specific goals 1 or more times.
As a fully developed concept, this feature would allow for inclusion or exclusion, along with compound AND/OR logic.
Obviously, a segment can be as complicated as we choose, but we are limited to 10 at any one time and most segmentation ideas occur during the analysis of an experiment, not always during the planning stage.
The ability to combine segments that are already created would expand the pool and give us the freedom to apply new segmentation as inspiration hits during the course of an experiment.