The One could not control the Elven rings, as those were made without the help of Sauron. But even just controlling the Nazgul would confer a substantial advantage in any battle.
However, I suspect Boromir and Denethor didn't really know what the ring could do for them concretely; they knew that it had powerful magic and everyone was saying that if Sauron had it there would be no stopping him - so surely there was some way for them to use that power, for good of course? (of ourse there is a way, my precious!)
Their thinking was mainly driven by the intense desire to help their country, to restore Gondor to its former glory. The ring used that desire against them.