For instance, I assume downloaded content must ‘expire’ after a certain period of time.
Using iTunes as an example, if I were to download a movie I would only have two or three days to watch it if I had started playing it. Otherwise I might keep it for months, despite that being an exercise in silliness (never mind a waste of valuable hard drive real estate).
Then there are the rights issues that accompany everything that appears on the streaming service. To get the aforementioned rights to television shows or movies Nextlix makes deals with content creators for millions of dollars, so does the ability to download their content cost Netflix more than they are currently paying (and this question is crucial, because if it does the likelihood is high that those costs would be passed down to consumers in the form of higher membership fees).
And while there has been no mention of any such increases–especially since the last time there was talk of a price increase it literally cost them millions of subscribers–nevermind the dubious ‘Quickster’ episode–we won’t know till we know.