In this paper we show that non-maximal coherence can lead to more efficient nonlinear frequency conversion than maximal coherence does. For this purpose we exemplify with the three-level cascade system (with no maximal coherence) to calculate the intensity of nonlinear-optical signal. A comparison is made between the Ξ system and the Λ system (with maximal coherence). The generated intensity is calculated by employing a nonperturbative approach. It is shown that under the conditions of the two-photon resonance and the same parameters, the largest intensity from the Ξ system is about twice as large as that from Λ system. The difference is analyzed in terms of dressed states. It is shown that both the spontaneous emission on the coupling transition and the spontaneous emission mediated three photon process play a contrary role for the two systems. These two processes enhance the generation of nonlinear signals for Ξ system but act contrarily for the Λ systems.