If you have empty spots on your roster, you’re killing your chances of making the playoffs. This is one of the worst things you can do! It’s far more common than it should be, but people will drop players and not pick up a replacement! Having a full roster has only benefits and no downsides. Every player on your roster is one that another owner cannot acquire.
Even if there isn’t anyone on the free agent list that you’re just dying to get, at least add someone. You will feel like a total dope if you decide against adding someone, and then the guy you thought about adding scores 3 touchdowns. Your chances of getting him just got much smaller. Also, check to see if you have an IR spot. If so, pick up a player who is injured but will be coming back later in the year. Keep that roster full!
I would also advise against having a backup kicker or defense. I would never have a backup kicker on my team. A backup defense is okay if your regular starting defense is on a bye (even still, I would personally not roster two defenses unless your main defense is absolutely ELITE–see Sacksonville).