Why do themes stack?

[Tory S. Anderson]

Throughout my work day I'll switch themes now and again to go from high 
contrast to lower, or from dark to light. This process requires disabling the 
present theme and then loading the next theme; if you forget to disable, the 
themes stack, usually with undesirable consequences. 

Why is it implemented this way? Does anyone out there actually gain utility 
from theme stacking? Or am I doing something wrong? I would think the simpler 
implementation would simply to have "load theme" automatically disable the 
present theme, which is both conceptually and pragmatically easier. But the 
manual doesn't seem to mention much about themes, so I haven't found an 
explanation (or possible use) for this stacking implementation.