Granted, it won't effect everyone and depends on optimum holding position and balancing it in your left hand. As the left margin is designed only to take you back a page, I found that it required some juggling to ensure your thumb fully extended to either tap the 'page fwd' zone or was just plain awkward to swipe from right to left, using only your left digits. Using right handed was far easier, as I tend to move forward (as you do...), rather than moving back a page constantly.
Again, if you use it holding in your left hand and can occasionally free your right hand to tap forward, not a problem. But if your right hand is occupied (grabbing the pole), then it's a question of timing. For me, the slimmer bezel on both the newer Kindle models gives you less to grip and hence, would never consider a trade in for these models.