It's important to separate downloading for storage and future use and downloading for real-time streaming.
If downloading for storage, the data rate doesn't matter except that it impacts the total amount of time for the download process.
If downloading for real-time streaming, a minimum data rate (8/6/2.5) is the ideal situation since that ensures that data will arrive at least as fast as it's consumed. If the actual rate is below that minimum, there is a delay prior to start to allow a buffer to build up a reserve.
In any event, it's possible that the buffer contents will empty prior to the movie's completion. If that occurs, the movie will stop playing until the buffer is replenished. Hence, stop/start during the play. I'm suspecting that the 1Mbps rate that you mentioned is the minimum required to prevent an unacceptable amount of stop/starts.