Potential risks of sitting upright on the saddle during a daily commute
Of course, you will have less control over your bike if you don't hold the handle bars, especially if something unexpected happens (like a truck coming out of nowhere at full speed for example).

Regarding sitting upright, you are very lucky to be able to keep your back straight like that, most people have a bad posture, and are not comfortable sitting upright on bar stools for example (or any chair without something to hold your back). Being upright like that uses your lower back muscles and your core more generally, which is a very good thing. Also you will relieve potential stress in your forearms, because they are not resting on the bar (with your weight).