Both sides of the graft retain their genetics... but they share sap. Consider what that could mean.
I agree that there is no point to grafting if a vegetable already does well unadulterated, other than curiosity & scientific experimentation. If I were to attempt grafting annuals, it would be to add or augment a new trait, such as heat/cold tolerance, insect/disease resistance, etc. Who knows, something unexpected might come out of the graft, such as a reduction in blossom end rot, or higher sugar content... or something negative, like bitterness.

It could also prove to be a way to provoke photoperiod sensitive plants to perform outside their normal range. That uncertainty is what makes experimenting fun. I certainly wouldn't put a lot of eggs into that basket, though.