An IRT framework allows one to explicitly model the relationship between item scores and the phenotype of interest. Any combination of items can of course be summed (weighted or unweighted), but this does not guarantee that the sum score reflects a meaningful construct. The meaningfulness of the measurement can be directly assessed in an IRT framework. Fit to an IRT model is empirical evidence that the observed responses can be explained by an underlying structure. The latent variable of the IRT model should, of course, be an appropriate representation of the construct to be measured.
