Do you need a sub? no. But even if you were buying the biggest baddest most expensive main speakers that you could find the system would sound better if it included a sub.
If you don't have a decent receiver to go with those speakers, then you will NEED a sub. Large drivers draw a lot of current to reproduce bass, which means an average receiver will struggle on bass heavy music and definitely with movies, which in turn will likely result in clipping (distortion) and possible speaker damage.
A sub, with it's own power, and the receiver crossed over to limit bass sent to the mains, will help out with power demands on the receiver and will almost certainly give you better bass response.