The bimetallic argument was basically a fight between those who already were wealthy and wished to preserve their position, and those who were not yet wealthy but wanted to improve their position.
Backers of a gold-only standard (mostly in the eastern US) wanted it because their fortunes were already amassed, and denominated in gold or gold-backed currency, and they did not want this diluted by the influx of silver from mines in the western US. Backers of the bimetallic standard had access to silver mines, or to the resulting silver, and wanted the ability to either have silver coined, or to present it and receive in return legal tender (which had been taken away in 1873).
For an edifying take on the populist/bimetallic position, consider reading William Jennings Bryan's "Cross of Gold" speech, which is full of eminently-quotable and still-relevant lines. For example, a rebuttal of what came to be known as the "trickle-down" theory:
Mr. Carlisle said in 1878 that this was a struggle between "the idle holders of idle capital" and "the struggling masses, who produce the wealth and pay the taxes of the country"; and, my friends, the question we are to decide is: Upon which side will the Democratic party fight; upon the side of "the idle holders of idle capital" or upon the side of "the struggling masses"? That is the question which the party must answer first, and then it must be answered by each individual hereafter. The sympathies of the Democratic party, as shown by the platform, are on the side of the struggling masses who have ever been the foundation of the Democratic party. There are two ideas of government. There are those who believe that, if you will only legislate to make the well-to-do prosperous, their prosperity will leak through on those below. The Democratic idea, however, has been that if you legislate to make the masses prosperous, their prosperity will find its way up through every class which rests upon them.