If we use multiple independent channels on which same information is transmitted at a same time, then we creating additional spatial degree of freedom that improves the signal strength and will have good coverage,etc. With diversity we improve the reliablity of the signal.
A single word gain cannot simply be used to describe the improvement of the system either in the sense of full diversity or full degree of freedom, there are so many other parameter. There can be a tradeoff between two sets of variable. One could have a good coverage, but might have low bandwidth of signal. Other could have a low coverage, but might have high bandwidth of signal.There is a tradeoff between error probability and transmission rate.Signal to Noise Ratio(SNR) always come into play with wireless systems.Diversity and Multiplexing gain are inversly proportional to each other. With increase of diversity there are is decrease in multiplexing gain. So again, we have a tradeoff. It depends what sets of parameter suits the best in a system. It's like you are keeping one value at extreme that is achieving full diversity. So multiplexing gain is going to be at the least possible value.