When comparing two things on one small screen, I want to be able to rapidly switch between two windows. Depending on if it's within same app, I have to think to use either Cmd+Tab or Cmd+`. And all the minimized apps, as well as apps on other desktops get in the way.
Mission Control is unacceptable for rapid window switching because it requires me to move the mouse pointer and find the other window, when all I want to do is "select previous window".