Originally Posted by chaseter
Optimus wears it when he is in battle and it looks as if he gets into 2 or 3 fights this round so he will be wearing it much more. Bay and the designers chose to not have it on constantly so that we could see his facial movements when he talked. That gives these characters tons of life and makes them more relatable. The facial cues tells a lot about each character as well and also helps the general audience know which robot is talking. So, in terms of design, I think it was a fantastic choice to not make him wear the faceplate the entire time. Even in the cartoons, the faceplate got bigger and even light up a little to show that he was talking.
Thing is, I think the old principal about eyes being perhaps the most important body part when it comes to actors emoting is something that I think could've easily been applied to Optimus.