I've never ridden a fixie so take this FWIW ... but I really thought that the fitness benefits of fixies came from (1) never being able to coast, and (2) using the antagonist leg muscles to slow down. You don't get either with a singlespeed. As far as smoothing out your pedal stroke, a set of rollers is way cheaper than redoing your drivetrain (or buying another bike) - and likely more effective for that purpose, too.

You could always get one of those reversible rear wheels that has a fixed cog on one side and a single-speed freehub on the other, so you could ride fixed when you wanted, but without the commitment.