The semileptonic B ->pi l(+)nu(l) decay is studied starting from a simple quark model which includes the influence of the B-* pole. To extend the predictions of a nonrelativistic constituent quark model from its region of applicability near q(max)(2)=(m(B)-m(pi))(2) to all q(2) values accessible in the physical decay, we use a novel multiply subtracted Omnes dispersion relation, which considerably diminishes the form-factor dependence on the elastic pi B ->pi B scattering amplitudes at high energies. By comparison to the experimental branching fraction we extract vertical bar V(ub)vertical bar=0.0034 +/- 0.0003(exp)+/- 0.0007(theory). To further test our framework, we also study D ->pi and D -> K decays and find excellent results f(pi)(+)(0)/f(K)(+)(0)=0.80 +/- 0.03, B(D-0->pi(-)e(+)nu(e))/B(D-0-> K(-)e(+)nu(e))=0.079 +/- 0.008. In particular for the D ->pi case, we reproduce, with high accuracy, the three-flavor lattice QCD results recently obtained by the Fermilab-MILC-HPQCD Collaboration. While for the D -> K case, we successfully describe the data for f(+)(q(2))/f(+)(0) recently measured by the FOCUS Collaboration.