There are two sides to the equation. If you don't have a lefty on the roster then who are you going to give up to get one? I would rather work with what we got until a deal can be made to balance out the unit. If you can't get one in a deal that works for you then you work with the players you have. Acting out of desperation to fill a need is not usually better than working with what you got for the short term.
In a cap system you can't always perfectly address all your needs. How you manage your limitations is an issue that all organizations have to contend with. As I previously stated I would rather play the more talented right hand player on the left side than play the less talented leftie on that side. It's not a perfect situation but you work with what you got until better options are available.