Nerve cells most likely evolved for a different purpose, high speed communication. Adaptivity of this network improves fitness because even when you are in one body you don't know how big it is (it grows), so you need to send different signals for different size bodies. Also if you can link in extant sensors used for chemotaxic or phototaxic behaviour and use this information in the high speed network without having to re-evolve the behaviours, then you can gain fitness advantages.

I'm saying it smooths out the curves of the search space that evolution is moving in. Rather than having discontinuous jumps between the fitness of (lack of eye, no information processing for eye) and (light sensor, genetic adaption for processing the information from the light sensor), you get the step of (light sensor, some system that can do something with the information) in between. Getting both together is unlikely.

In this respect it plays a similar role to hox genes. Getting symmetrical legs for locomotion (or wings for flight) is unlikely unless you have a modular system.