In uplink block-fading multiple access channel (BF-MAC), the advantage of multiuser diversity (MUD) can be taken to achieve higher system throughput. In this letter, with rate constraints for all users and only channel state information available at the receiver assumed, we demonstrate that non-orthogonal multiple access (NOMA) outperforms the counterpart of orthogonal multiple access (OMA) via exploiting the MUD from the superposition of multiple users. The MUD gain achieved by NOMA compared with OMA is quantitatively analyzed for finite users, and the closed-form upper bound of MUD gain from the superposition of infinite users is also derived. Numerical results show that the potential MUD gain from superposition of infinite users can be well approached with limited superposed users, which indicates that multiple access schemes with limited superposed users can provide a good tradeoff between system performance and decoding complexity.