No, this never made a great deal of sense, keeping two lefty-hitting first baseman on the roster, both of them probably good enough to deserve at least half of a platoon role. So this news shouldn't be a huge surprise (via the Arizona Republic):

Now that the muddled first-base situation has been cleared up with the release of Russell Branyan, it opens the door for Juan Miranda to get the bulk of the playing time. Manager Kirk Gibson said it was time to move away from the three-man rotation that also included Xavier Nady.

--snip--

Gibson said no one knew how or when the first-base situation would work out. He said it was interesting to have three first basemen on the roster coming out of spring training, where Branyan earned his spot with a hot bat.

Juan Miranda is not great and will never be great. But he's perfectly decent and exceptionally cheap, certainly good enough to play against right-handed pitching for a rebuilding team like the Diamondbacks.

There's very little reason to think that Xavier Nady can still hit. Since missing most of 2009 with an elbow injury (and ultimately, Tommy John surgery) Nady's batted .259/.309/.357 in 418 plate appearances. Maybe he'll get it back, but Nady's 32 and with the exception of one season (2008) he's never really hit like a first baseman anyway.

Meanwhile, Branyan has typically hit like a first baseman. If you don't mind the strikeouts, Branyan's .245/.337/.515 line over the last three seasons looks suspiciously like someone who deserves a semi-regular job in the majors. With Adam LaRoche having a terrible season -- probably because of his shoulder injury -- the Nationals could use a guy like Branyan right now. And it might be almost time for the Padres to pull the plug on Brad Hawpe.