That is the common thought here at UAL, but interestingly enough I asked that specific question on the private UALpilotsfroum, and the answer was a bit surprising. The thread ran multiple pages with lots of discussion, but not once did someone mention reserve rules. The consensus from nearly everyone was that NBCA required way more work which was fine BUT the pay gap between WBFO and NBCA was not enough to entice people to leave WB flying. In fact quite a few comments came from individuals who were NBCAs and bid back to WBFO. Im sure it's much the same at Delta, but WB schedules are easy 85 hours for 12 days or less with many premium pay opportunities where as junior NB guys are typically pushed closer to 90 hours and 15-18 multi-leg days (even 4 leg days quite often on the Airbus) with the premium pay going to senior guys.