Is back-panel access really worth the extra zipper weight?

For most multi-day tourers, yes. Back-panel access allows you to retrieve gear from the main compartment without removing skis from the carry system. On a ridge transition or rest stop, this saves significant time and hassle—you set the pack down on its front (skis pointing up), unzip the back panel, and access everything. The alternative is detaching skis, opening the top, digging to the bottom, repacking, and reattaching skis. Over multiple days with multiple stops, the convenience is substantial. The zipper adds roughly 50–80g.