It just seems to me that when I solo the neck pup on my Jazz, I get pretty close to a P sound. So why not play a Jazz and have the option of both?
I don't know if you've ever compared the neck pickup on a jazz to a P-bass pickup in an ensemble setting, but they're not the same.. they poke through in different places... they both have that slightly 'hollow' midrangey sound but in practical use one's not a substitute for the other
that's why you (comparatively) rarely get people using neck only on a Jazz bass, and why people don't say "thank god I ditched my P and started using the a Jazz with the neck pickup soloed etc"

The one Jazz bass I have has a very old Warmoth neck that is a FULL C shape. So........for me the question is........Is a J bass necessary??