This question is not only about using both hands simultaneously, it's about being able to use either of your hands in general. By practical, I mean advantages and disadvantages in terms of drawing. In other words I'm not talking about [cut of aggression level](https://cogsci.stackexchange.com/a/4528) etc. 

I've started learning to draw recently using [Draw a Box](http://drawabox.com/) lessons and wondering whether I should do the offered exercises with both hands or just focus on my dominant hand. I read somewhere that some professional painters use their non-dominant hand for less skill demanding tasks, so their dominant hand gets less worn out or tired. Others say that they learn to use their non-dominant hand so they can use it for work in case they injure their dominant one.