To be honest Paul, the strain is only in the beginning. Once you have done it for a while, it becomes very easy and even 2nd nature. I'm so used to it now that I forget sometimes and converge my eyes when viewing a 2D image just out of habit. That doesn't mean I would watch a movie that way. I don't recommend prolonged freeviewing.
For those that don't like the crossview format, there are others where you don't have to cross your eyes. Each format however carries their own set of advantages and disadvantages.
If you have Red/Cyan filtered 3D glasses for instance, you could view this same image in an anaglyph format...