Of course you don't need zfw to compute t/o weight.
OK, you might not need it in a Cessna 172, but with longer aircraft cabins with multiple loading configurations, what starting point would you suggest we use to calculate takeoff CG?
For the aircraft that i fly, if the ZFW CG is within limits, then you cannot put it out of limits by adding fuel. So for us the ZFW, Moment and CG is extremely important.
Mutt