My rationale for Muncy is he has played a lot of 1B/2B/3B over his career, so you aren't necessarily blocking anybody unless both Jung and Keith bang down the door. He can go to where he is needed, and unlike some of the LH outfielders on the market, his platoon split is neutral and he is a decent fit in that regard.
I do agree there is decline risk, but he's been a reliable 2-5 win player the last six years.