Most games in uma0 don't save properly. This is likely because games themselves don't recognize the path they're installed to (as some games do work properly in uma/ur0, but not very many), so games can't create/read save files.
You can use a homebrew to switch the mount points between the SD2Vita and official memory card on the fly though, so you can add games to both and just swap the mount points back and forth to play whatever game.
As to the OP, sure. If SD2Vita stops working, breaks, bricks, you'll always have an official backup that won't have issues. You can use it strictly for storing Adrenaline and PSP/PS1 games/Vita homebrew mounted as uma0, which is usually the accepted use for it.