You are correct; out-of-the-box P6 does not calculate Earned Value (EV) at the resource assignment level. It only runs EV at the activity level which is fine for a start, but what to do when you need to ‘dig deeper’ and analyze crew performance? You can't use Global Change to calculate the resource assignment EV because the activity percent complete isn't available in the Global Change list of fields.
A very simple option is to use TAPS to input your schedule updates. TAPS is barcode updating (with additional functionality). By using TAPS, updates are faster and more accurate giving you (and your team) more time for schedule analysis. Yes, TAPS runs EV calculations at the resource level.